/  -a.    0   3 


LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 

Deceived 
^Accessions  No.O  .%2# .# .  . . . ..  Class  Nc . 


A  TEXT-BOOK 


ON 


ROADS  AND  PAVEMENTS. 


BY 


FRED    P.    SPALDING, 

Assistant  Professor  of  Civil  Engineering  in  Cornell  University* 
Member  A  tnerican  Society  of  Civil  Engineers. 


FIRST   EDITION. 
FIRST   THOUSAND. 


NEW  YORK : 

JOHN    WILEY    &    SONS, 

53  EAST  TENTH  STREET. 

1894. 


Copyright,  1894, 

BY 

FRED  P.  SPALDING. 


PREFACE. 


SUCCESSFUL  practice  in  the  construction  of  highways 
must  depend  upon  correct  reasoning  from  elementary 
principles  in  each  instance  rather  than  upon  following 
definite  rules  or  methods  of  construction. 

The  aim  of  this  book  is  to  give  a  brief  discussion, 
from  an  engineering  standpoint,  of  the  principles  in- 
volved in  highway  work,  and  to  outline  the  more  im- 
portant systems  of  construction,  with  a  view  to  form- 
ing a  text  which  may  serve  as  a  basis  for  a  systematic 
study  of  the  subject. 

Details  and  statistics  of  particular  examples  have 
for  the  most  part  been  excluded  as  undesirable  in  a 
book  of  this  character.  Such  information  is  available 
in  many  forms  for  those  having  the  necessary  ele- 
mentary training  and  experience  to  enable  them  to 
properly  use  it. 

Considerable  space  has  been  given  to  the  location 
and  construction  of  country  roads,  as  seemed  proper 
in  view  of  the  present  general  public  interest  in  the 
matter,  and  the  probable  development  of  this  new  field 
of  activity  in  engineering  work.  The  improvement  of 
our  common  roads  must  come  through  transferring 
such  work  to  the  charge  of  those  who  make  it  a  profes- 
sion, and  not  through  teaching  the  public  how  roads 
should  be  constructed. 

F.  P.  S. 

ITHACA,  N.  Y.,  July,  1894. 

iii 


THTIVBE: 


CONTENTS. 

CHAPTER   I. 

GENERAL    CONSIDERATIONS. 

PAGE 

Art.     i.  Object  of  Roads i 

2.  Resistance  to  Traction 2 

3.  Tractive  Power  of  Horses 7 

4.  Desirability  of  Various  Surfaces 9 

5.  Economic  Value 1 1 

6.  Healthfulness 14 

7.  Safety 16 

8.  Durability 18 

CHAPTER   II. 

DRAINAGE   OF   STREETS   AND    ROADS. 

Art.     9.  Necessity  for  Drainage 22 

10.  Surface  Drainage 23 

11.  Subsurface  Drainage 26 

12.  Kinds  of  Soil 28 

13.  Types  of  Drains 31 

14.  Culverts 34 

1 5.  Water-breaks 42 

CHAPTER   III. 

LOCATION   OF   COUNTRY  ROADS. 

Art.   16.  Considerations  governing  Location 43 

17.  Length  of  Road 47 

v 


VI  CONTKNTS. 


Art.   18.  Rise  and  Fall 49 

19.  Rate  of  Grade -. 51 

20.  Examination  of  Country 53 

21.  Placing  the  Line 56 

22.  Comparison  of  Routes 59 

23.  Changing  Existing  Locations 63 

CHAPTER   IV. 

IMPROVEMENT   OF  COUNTRY   ROADS. 

Art.  24.  Nature  of  Improvements 67 

25.  Earthwork 68 

26.  Drainage 72 

27.  Earth-road  Surface 73 

28.  Gravel  Roads 76 

29.  Maintenance  of  Country  Roads 78 

30.  Width  of  Country  Roads 80 

31.  Economic  Value  of  Road  Improvements 82 

32.  Systems  of  Road  Management 85 

CHAPTER  V. 

BROKEN-STONE   ROADS. 

Art.  33.  Definition , 88 

34.  Macadam  Roads 89 

35.  Telford  Foundations 91 

36.  Choice  of  Foundation 93 

37.  Materials 96 

38.  Binding  Material 101 

39.  Compacting 103 

40.  Thickness  of  Road  Covering 105 

41.  Cross-section 107 

42.  Maintenance 107 

CHAPTER  VI. 

FOUNDATIONS   FOR   PAVEMENTS. 

Art.  43.  Preparation  of  Road-bed no 

44.  Purpose  of  Foundation 1 1 1 


CONTENTS.  Vll 

PAGE 

Art.  45.  Sand  Foundation 112 

46.  Gravel  and  Broken  Stone 113 

47.  Concrete 114 

48.  Brick 116 

49.  Sand  and  Plank 117 

50.  Depth  of  Foundation 117 

CHAPTER  VII. 

BRICK  PAVEMENTS. 

Art.   51.   Paving  Brick 119 

52.  Tests  for  Paving  Brick 122 

53.  Foundations 128 

54.  Construction 1 29 

55.  Maintenance 132 

CHAPTER  VIII. 

ASPHALT   PAVEMENTS. 

Art.   56.  Asphaltum 134 

57.  Rock  Asphalt 140 

58.  Asphalt  Blocks 142 

59.  Foundations 144 

60.  Construction 146 

61.  Vulcanite  or  Distillate  Pavement 148 

62.  Maintenance 149 

CHAPTER   IX. 

WOOD   PAVEMENTS. 

Art.  63.  Wood  Blocks 151 

64.  Foundations 1 54 

65.  Construction 1 56 

66.  Preservation  of  Wood 161 

67.  Maintenance 164 

68.  Healthfulness 165 


Vlll  CONTENTS. 

CHAPTER   X. 

STONE-BLOCK   PAVEMENTS. 

PAGE 

Art.  69.  Stone  for  Pavements 168 

70.  Cobblestone  Pavements 170 

71.  Belgian  Blocks 172 

72.  Granite  and  Sandstone  Blocks 172 

73.  Construction 173 

74.  Stone  Trackways 176 

CHAPTER   XI. 

CITY   STREETS. 

Art.  75.  Arrangement  of  City  Streets 178 

76.  Width  and  Cross-section 183 

77.  Street  Grades 187 

78.  Street  Intersections 189 

79.  Footways 191 

80.  Curbs  and  Gutters 196 

81.  Crossings 202 

82.  Street-railway  Track 203 

83.  Trees  for  Streets 211 

84.  Alleys 212 


• 


A  TEXT-BOOK 

ON 

ROADS   AND    PAVEMENTS 


CHAPTER   I. 
GENERAL   CONSIDERATIONS. 

ART.  i.    OBJECT  OF  ROADS. 

THE  primary  object  of  a  road  or  street  is  to  provide 
a  way  for  travel,  and  for  the  transportation  of  goods 
from  one  place  to  another.  The  facility  with  which 
traffic  may  be  conducted  over  any  given  road  depends 
upon  the  resistance  offered  to  the  passing  of  vehicles 
by  the  surface  or  the  grades  of  the  road,  as  well  as 
upon  the  freedom  of  movement  allowed  by  the  width 
and  form  of  the  roadway.  In  order  that  a  road  may 
offer  the  least  resistance  to  traffic,  it  should  have  as 
hard  and  smooth  a  surface  as  possible,  while  affording 
a  good  foothold  to  horses,  and  should  be  so  located  as 
to  give  the  most  direct  route  with  the  least  gradients. 

The  expediency  of  any  proposed  road  construction 
or  improvement  depends  upon  its  desirability  as  affect- 
ing the  comfort,  convenience,  and  health  of  residents  of 


2          A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

the  locality,  and  also  upon  its  economic  value  which  is 
largely  determined  by  its  cost  and  durability,  as  well 
as  upon  the  facility  it  gives  for  the  conduct  of  traffic. 

The  problem  of  the  highway  engineer,  in  designing 
works  of  this  character,  involves  the  consideration  of 
these  various  elements  and  their  proper  adjustment  to 
give  the  best  results. 

The  kinds  of  road  surface  most  commonly  employed 
are  as  follows  :  For  the  streets  of  cities  and  towns, 
pavements  of  stone  blocks,  brick,  asphalt,  and  wood  ; 
for  suburban  streets  and  important  country  roads, 
macadam  and  gravel  surfaces  ;  for  ordinary  country 
roads  in  general,  surfaces  of  earth  or  gravel. 

ART.  2.     RESISTANCE  TO  TRACTION. 

The  resistance  to  traction  of  a  vehicle  on  a  road 
surface  may  be  divided  into  three  parts  :  axle  friction, 
rolling  resistance,  and  grade  resistance. 

Axle  friction  varies  with  the  nature  of  the  bearing 
surfaces,  and  for  vehicles  of  similar  construction  is  di- 
rectly proportional  to  the  load.  It  is  entirely  inde- 
pendent of  the  nature  of  the  road  surface. 

Rolling  resistance  is  of  two  kinds  :  that  due  to  irregu- 
larities in  the  surface  of  the  road,  and  that  of  a  wheel 
to  rolling  upon  a  smooth  surface,  sometimes  called 
rolling  friction. 

The  resistance  due  to  an  inequality  in  the  road  sur- 
face, is  the  horizontal  force  necessary,  at  the  axle,  to 
raise  the  weight  upon  the  wheel  to  the  height  of  the 
obstacle  to  be  passed.  Thus  (Fig.  i),  by  the  principle 

of  the  lever,  P=  W. 


GENERAL  CONSIDERATIONS.  3 

For  small  inequalities,  this  resistance  will  be  approxi- 
mately inversely  as  the  diameter 
of  the  wheel.  The  effect  of  small 
irregularities  in  the  surface,  how- 
ever, is  due  more  to  the  shocks  and 
concussions  produced  by  them  than 
to  the  direct  lifting  action  of  the 

obstacle,  and  the  resistance  due  to      

uneven   surface  is  greater  at  high  ~ 

than  at  low  velocities. 

Rolling  friction  is  probably  due  for  the  most  part  to 
the  compressibility  of  the  surface  of  the  road,  which 
permits  the  wheel  to  indent  it  to  some  extent.  The 
wheel  is  thus  always  forcing  a  wave  of  the  surface 
before  it,  or  climbing  an  inclination  caused  by  its 
weight  upon  the  road  surface.  This  rolling  friction 
varies  for  wheels  of  differing  diameters,  being  less  for 
large  than  for  small  wheels.  The  experiments  of  M. 
Morin,  in  France,  seemed  to  indicate  that  the  resist- 
ance varies  inversely  as  the  diameter.  Other  experi- 
ments have  indicated  a  less  variation,  approximately 
as  the  square  root  of  the  diameter,  while  Mr.  D.  K. 
Clark  (Roads  and  Streets  by  Law  and  Clark ;  London, 
1890)  concludes,  from  a  mathematical  discussion  based 
upon  the  assumption  that  the  material  of  the  surface 
is  homogeneous  and  the  pressure  proportional  to  the 
depth  of  penetration,  that  the  resistance  to  traction 
is  inversely  as  the  cube  root  of  the  diameter  of  the 
wheel. 

For  practical  purposes  it  may  be  considered  that,  for 
wheels  of  ordinary  sizes  used  on  road  vehicles,  the 
rolling  resistances  are  equal  to  the  load  multiplied  by  a 
coefficient  which  depends  upon  the  nature  and  condi- 


4          A  TEXT-BOOK   ON    ROADS  AND   PAVEMENTS. 

tion  of  the  road  surface,  although  these  coefficients 
are  somewhat  affected  by  the  sizes  of  the  wheels. 

Many  experiments  have  been  made  for  the  purpose 
of  determining  the  tractive  force  required  for  a  given 
load  upon  various  road  surfaces.  The  results  show 
somewhat  wide  variations,  as  would  be  expected  when 
the  many  elements  that  may  affect  them  are  con- 
sidered. The  following  table  shows  a  few  average 
results,  which  will  give  some  idea  of  the  relative  resist- 
ances of  various  surfaces  and  of  the  advantage  to  be 
derived  from  a  smooth  and  well-kept  road  surface  : 

TRACTIVE   RESISTANCES   ON   VARIOUS    SURFACES. 

Character  of  Road.  Resistance,  Lbs.  per  ton. 

Earth — ordinary  in  fair  condition.  125  to  140 

dry  and  hard 60  "  100 

Macadam — very  good 40  "  60 

ordinary 60  "  80 

poor 100  "  150 

Granite-block  pavement — good. . .  25  •  40 

ordinary  50  "  80 

Asphalt  pavement « 15  "  25 

Wooden-block  pavement 20  "  30 

On  earth  roads  or  smooth  pavements  the  tractive 
force  is  independent  of  the  velocity ;  but  on  rough 
pavements,  where  concussions  take  place,  the  tractive 
force  increases  as  the  speed  increases. 

Grade  Resistances. — Tractive  resistance  due  to  grade 
is  independent  of  the  nature  of  the  road  surface,  or  of 
the  size  of  the  wheels.  It  is  equal  to  the  load  multi- 
plied by  the  sine  of  the  angle  made  by  the  grade  with 


GENERAL   CONSIDERATIONS. 


5 


the  horizontal.     Thus  in  Fig.  2  the  tractive  force  P, 

due  to  the  grade,  is  the  force  necessary  to  prevent  the 

wheel  from  rolling  down 

the  slope   under  the  ac-  /    ™\       \      P 

tion  of  the  weight  W,  or 

it   is   the    component  of 

^parallel  to  the  slope  ac. 


ac 


FIG.  2. 


Grades  are  ordinarily  expressed  in  terms  of  rise  or 
fall  in  feet  per  hundred,  or  as  percentage  of  horizontal 
distance. 

For  all  ordinary  cases  of  small  inclinations  ab  is 
approximately  equal  to  ac,  and  we  may  take 

P-  W-- 

* ab' 

or  the  tractive  force  necessary  to  overcome  any  grade 
equals  the  load  multiplied  by  the  percentage  of 
grade. 

The  total  tractive  force  necessary  to  haul  a  load  up 
an  inclined  road  equals  the  sum  of  the  force  necessary 
to  haul  the  load  upon  the  same  surface  when  level,  and 
the  force  necessary  to  overcome  the  grade  resistance. 
Thus,  if  we  wish  to  find  the  tractive  effort  necessary  to 
haul  a  load  of  2  tons  up  a  grade  of  3  ft.  in  100  over  a 
good  macadam  road.  Taking  the  resistance  of  the 
road  surface  when  level  at  60  Ibs.  per  ton,  we  have  for 
the  total  resistance 

R  =  2  X  60  +  4000  X  rib-  =  240  Ibs. 


6          A  TEXT-BOOK   ON   ROADS  AND    PAVEMENTS. 

In  going  down  the  grade,  the  force  due  to  grade 
becomes  a  propelling  force,  and  the  tractive  effort 
required  is  the  difference  between  the  surface  resist- 
ance and  grade  force.  In  case  the  grade  force  be  the 
greater,  the  resulting  tractive  force  becomes  negative, 
or  it  will  be  necessary  to  apply  the  force  as  a  resistance 
to  prevent  acceleration  of  the  velocity  in  the  descent. 

The  angle  for  which  the  tractive  force  required  for  a 
given  surface  equals  the  grade  resistance  is  called  the 
Angle  of  Repose  for  that  surface.  In  the  case  given 
above,  2  X  60  —  4000  X  yjru-  =  o,  or  the  angle  of  repose 
for  a  surface  whose  level  resistance  is  60  Ibs.  per  ton  is 
a  3$  grade.  If  a  vehicle  were  left  standing  upon  that 
inclination,  it  should  remain  standing  with  the  forces 
just  balanced.  If  it  were  started  down  the  grade,  it 
should  continue  to  move  at  a  uniform  rate,  without 
the  application  of  any  other  force. 

In  a  series  of  experiments  made  by  the  Studebaker 
Brothers  Manufacturing  Company  (see  The  Engi- 
neering Record  for  Dec.  16,  1893)  upon  the  traction 
necessary  upon  various  surfaces  with  American  wagons, 
it  was  found  that  the  width  of  wheel-tire  has  little  if 
any  effect  upon  a  hard  surface  ;  that  there  was  a  small 
difference  in  favor  of  the  wide  tire  upon  soft  ground, 
and  upon  sod  the  narrow  tire  would  cut  through  where 
the  wide  one  would  pass  over. 

The  wheels  used  in  the  tests  had  tires  if,  3,  and 
4  inches  wide,  and  varied  from  3  ft.  6  in.  to  4  ft.  6  in. 
in  diameter. 

The  general  results  show  a  variation  in  tractive  force 
required,  depending  upon  the  construction  of  the 
vehicle,  of  from  30  to  65  pounds  per  ton  for  a  stone- 
block  pavement,  from  120  to  175  pounds  per  ton  for  a 


GENERAL  CONSIDERATIONS.  7 

good  sand  road,  from  60  to  100  pounds  per  ton  for  a 
gravel  road,  and  from  240  to  325  pounds  per  ton  on  a 
muddy  road. 

It  was  also  found  that  the  force  necessary  to  start 
the  load  was  from  125  to  200  pounds  per  ton  greater 
than  that  necessary  to  keep  it  in  motion  ;  the  load 
starting  easier  on  wheels  of  large  diameter  than  upon 
small  ones,  but  the  diameter  seemingly  having  less 
effect  upon  the  traction  when  in  motion. 


ART.  3.    TRACTIVE  POWER  OF  HORSES. 

The  loads  that  a  horse  can  pull  upon  various  road 
surfaces  will  not  necessarily  be  proportional  to  the 
resistance  offered  by  the  surface  to  traction,  as  the 
tractive  force  that  the  horse  can  exert  depends  upon 
the  foothold  afforded  by  the  surface.  The  ability  of  a 
horse  to  exert  a  tractive  force  depends  upon  the 
strength  of  the  animal,  upon  his  training  for  the  par- 
ticular work,  and  whether  he  be  accustomed  to  the 
surface  upon  which  he  is  travelling.  The  work  of  dif- 
ferent animals  is  therefore  subject  to  considerable  varia- 
tions, and  only  very  rough  approximations  are  possible 
in  giving  average  values  of  the  work  a  horse  may  do 
under  differing  conditions. 

The  tractive  force  that  may  be  exerted  by  a  horse, 
at  moderate  speeds,  varies  approximately  inversely  as 
the  rate  of  speed  ;  or,  in  other  words,  the  power  that  a 
horse  can  exert  through  any  considerable  time  is  nearly 
constant  for  varying  velocities.  Thus  it  may  be  as- 
sumed, as  an  average  value,  that  a  horse  working 
regularly  ten  hours  per  day  can  put  forth  a  tractive 


8          A   TEXT-BOOK    ON   ROADS   AND    PAVEMENTS. 

effort  of  80  pounds  at  a  speed  of  250  feet  per  minute 
on  an  ordinary  level  road  surface. 

For  the  power  of  the  horse  we  then  have 

Power  —  force  X  velocity  =  80  X  250  =  20000  foot-lbs. 
per  minute. 

For  any  other  rate  of  speed,  as  200  feet  per  minute, 
we  would  have  20000  -f-  200  =  100  pounds  as  the 
tractive  force  exerted  by  the  horse. 

If  the  period  of  daily  work  be  lessened,  the  power 
that  may  be  developed  will  be  increased,  either  by  in- 
creasing the  load  or  the  velocity. 

The  tractive  force  that  a  horse  is  able  to  exert  de- 
creases very  rapidly  as  the  rate  of  inclination  increases. 
This  is  due  both  to  the  expenditure  of  power  by  the 
horse  in  lifting  his  own  weight  up  the  grade,  and  to 
the  less  firm  footing  on  the  inclination.  The  effect  of 
differences  in  the  foothold  afforded  by  various  pave- 
ments is  very  marked  in  the  loss  of  tractive  power 
upon  grades. 

In  the  table  below  are  given  the  loads  that  an 
average  horse  may  be  expected  to  continuously  haul 
up  different  inclinations,  on  various  road  surfaces,  at 
slow  speed.  These  figures,  while  of  little  value  as  an 
absolute  measure  of  what  may  be  done  in  any  par- 
ticular case,  are  of  use  as  a  rough  comparison  of  the 
relative  tractive  properties  of  different  surfaces  and 
grades.  The  effect  of  grades  upon  tractive  effort  will 
also  depend  upon  the  condition  in  which  the  surface  is 
maintained,  and  upon  the  weather.  Snow  and  ice  in 
winter,  or  the  damp  and  muddy  condition  of  some 
pavements  in  wet  weather,  have  a  very  considerable 
effect  to  diminish  tractive  power. 


GENERAL   CONSIDERATIONS. 


LOADS  IN   POUNDS    THAT   A   HORSE    CAN   DRAW    UPON 
VARIOUS   SURFACES   AND    GRADES. 


Kinds  of  Surface. 

Rate  of  Grade. 

Level. 

i  in 

IOO. 

2  in 

IOO. 

3  in 

IOO. 

4  in 

IOO. 

Sin 

TOO. 

10  in 

IOO. 

15  in 

IOO. 

Earth  road  —  good    

3000 
1300 
4000 
1600 
6000 
3000 

10000 

2400 

IIOO 

2700 

IIOO 

4500 
2300 
4000 

2000 

900 
2OOO 
800 
3300 
1700 
2500 

1600 

700 
1600 
600 
2700 
1400 
1800 

1400 
600 
1400 

500 

2  2OO 
IIOO 

1300 

1200 

500 
I2OO 
450 
I7OO 
000 
1000 

800 
400 

700 
250 
900 
450 
400 

300 
150 
200 

IOO 

400 

200 

poor  
Broken-stone  —  good  

poor  

Stone  Blocks  —  good  
poor  
Asphalt—  clean  and  dry  

On  steep  grades  (more  than  8  or  10  in  100)  special 
forms  of  block  pavements  are  sometimes  employed  to 
increase  the  tractive  power  by  affording  better  foot- 
hold to  horses.  Sheet  asphalt  is  not  usually  employed 
on  grades  of  more  than  4$.  Ordinary  wood  blocks 
and  brick  are  used  up  to  grades  of  7%  or  8$,  and  granite 
blocks  to  10%. 

A  horse  may  frequently  exert  for  a  short  time  a 
tractive  force  about  double  that  which  he  can  exert 
continuously ;  hence,  when  short  grades  occur  steeper 
than  the  general  grades  of  the  road,  loads  may  often 
be  taken  over  them  much  heavier  than  could  be  carried 
if  the  steeper  grade  prevailed  upon  the  road. 

ART.  4.     DESIRABILITY  OF  VARIOUS  ROAD 
SURFACES. 

The  desirability  of  a  road  surface  for  any  particular 
use  depends  both  upon  its  fitness  for  the  service  re- 
quired of  it  and  upon  its  durability  in  use. 

Upon  a  country  road,  the  problem  of  improvement 
ordinarily  consists  simply  in  providing  the  hardest  and 


10       A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

most  durable  surface  consistent  with  an  economical 
expenditure  of  available  funds,  the  object  being  to 
lighten  the  cost  of  transportation  by  reducing  the 
resistance  to  traction,  and  to  render  travel  easy  and 
comfortable. 

Upon  city  streets,  however,  several  other  factors 
may  be  of  importance  in  the  design  of  highway  im- 
provements. 

The  comfort  both  of  those  using  the  street  and  of 
the  occupants  of  adjoining  property  will  be  largely 
affected  by  the  freedom  of  the  surface  from  noise  and 
dust. 

The  safety  of  the  pavement  in  use,  its  effect  upon 
the  health  of  residents  of  the  locality,  and  its  economic 
value  must  in  each  case  be  considered. 

To  adjust  to  the  best  advantage  these  various  ele- 
ments, frequently  quite  discordant  with  each  other,  is 
a  matter  which  can  only  be  accomplished  by  the  exer- 
cise of  good  judgment.  Local  conditions  and  necessi- 
ties must  always  be  considered — such  as  the  difficulties 
of  drainage,  the  availability  of  various  materials,  the 
nature  of  the  traffic  to  be  carried,  and  the  needs  of  the 
business  or  property  interests  of  the  neighborhood. 
Thus,  for  heavy  hauling  of  a  large  city,  the  durability 
and  resistance  to  wear  of  the  pavement  may  be  the 
paramount  consideration  ;  for  an  office  district,  quiet 
may  be  very  important ;  for  the  lighter  driving  of  a 
residence  street,  the  elements  of  comfort  and  health- 
fulness  may  properly  be  considered  as  of  greater  force 
than  the  purely  economic  ones ;  while  in  all  of  the 
cases  the  necessary  limitation  of  first  cost  will  largely 
determine  what  may  or  may  not  be  done. 


GENERAL  CONSIDERATIONS.  II 

ART.  5.     ECONOMIC  VALUE. 

The  determination  of  the  economic  value  of  any 
proposed  road  or  street  improvement  is  always  a 
matter  of  difficulty,  as  it  embraces  so  many  items 
which  cannot  be  exactly  evaluated.  The  factors  to  be 
considered  in  this  connection  are  : 

1.  Cost  of  construction. 

2.  Cost  of  maintenance  and  repairs. 

3.  Cost  of  conducting  transportation. 

4.  Effect  upon  land  values  or  business  interests. 
For  the  purpose  of  comparing  various   pavements, 

or  of  considering  the  advisability  of  any  proposed  im- 
provement, we  may  sum  the  interest  on  the  cost  of 
construction  with  the  annual  charges  representing  the 
other  items,  and  find  which  improvement  will  make  the 
total  annual  cost  a  minimum  or  annual  benefit  a  maxi- 
mum. This  process,  in  any  case  in  practice,  simply 
amounts  to  a  use  of  the  judgment,  having  properly  in 
view  the  various  interests  to  be  affected,  as  to  what 
expense  may  legitimately  be  allowed  in  order  to  secure 
a  certain  benefit.  The  outlay  is  usually  quite  tangible 
and  easily  estimated,  while  the  advantages  cannot  be 
directly  estimated  and  are  often  overlooked.  It  must 
not,  however,  be  supposed  that  they  have  no  financial 
value,  or  that  the  pavement  which  can  be  constructed 
and  maintained  for  the  least  money  is  necessarily  the 
most  economical  to  use. 

77^^   cost   of  construction    must,    of   course,    include 
everything  connected  with    the  original    construction 
of  the  road  and  all  necessary  expenses  leading  to  tl 
improvement  under  consideration. 

The  cost  of  maintenance  and  repairs  includetf'an  esti- 


12        A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

mate  of  the  average  cost  of  keeping  the  road  in  good 
condition  over  a  term  of  years,  taking  into  account  the 
necessity  of  renewing  the  surface  at  the  expiration  of 
the  life  of  the  pavement,  the  cost  of  cleaning  and 
sprinkling,  and  such  minor  repairs  as  may  be  neces- 
sary from  time  to  time  to  maintain  a  uniformly  good 
surface. 

An  approximate  estimate  upon  these  points  may 
usually  be  made  by  examining  the  records  of  the  same 
kind  of  construction  under  similar  conditions  else- 
where. The  cost  of  maintenance  of  each  kind  of 
pavement  varies  widely  in  different  localities  and 
under  differing  treatment,  and  no  general  rules  can  be 
stated  as  to  the  relative  costs  of  the  various  systems. 

All  road  surfaces  will  require  maintenance,  the  same 
as  any  other  class  of  engineering  constructions  sub- 
jected to  wear  in  use ;  and  as  a  rule  the  cost  of  main- 
tenance will  be  less  as  the  care  used  in  keeping  the 
surface  always  clean  and  in  good  condition  is  greater. 

The  cost  of  transportation  is  affected  by  the  nature 
and  condition  of  the  road  over  which  the  traffic  must 
pass,  both  because  the  resistance  to  traction  offered  by 
the  surface  determines  the  load  that  may  be  hauled 
over  it,  and  because  the  roughness  of  the  surface 
serves  both  to  limit  speed  and  to  cause  wear  upon 
horses,  harness,  and  vehicles.  The  evaluation  of  these 
items  is  a  matter  of  difficulty,  on  account  of  the  practi- 
cally indeterminate  nature  of  the  data  upon  which  they 
should  be  based. 

A  rough  idea  of  the  relative  cost  of  transportation 
over  different  road  surfaces  may  sometimes  be  obtained 
by  observing  or  estimating  the  extent  and  nature  of 
the  traffic  that  is  likely  to  pass  over  the  road  and  esti- 


GENERAL   CONSIDERATIONS.  13 

mating  the  cost  of  carrying  this  traffic  over  each  sur- 
face. The  portion  of  the  traffic  which  consists  in 
hauling  maximum  loads  will  be  directly  affected  by 
differences  in  tractive  resistances  ;  the  number  of  loads 
necessary  to  move  the  traffic,  and  hence  the  cost, 
being,  for  this  portion,  approximately  proportional  to 
the  resistance.  For  the  lighter  portion  of  the  traffic 
the  greater  speed  with  a  smooth  surface  and  easy 
grades  will  be  of  value  in  the  saving  of  time,  although 
difficult  to  state  in  money  values. 

The  effect  of  a  smooth  surface  is  also  very  appre- 
ciable in  the  cost  of  wear  and  tear  upon  horses,  vehi- 
cles, and  harness.  The  value  of  this  item  is  variously 
estimated,  and  probably  ranges  from  one  to  ten  cents 
per  mile  travelled. 

Earth  roads,  in  good  condition,  and  wood  pavements 
seem  most  favorable  to  horses,  although  asphalt  and 
broken-stone  roads  are  commonly  considered  most 
advantageous  as  to  general  wear.  Brick  would  not 
differ  greatly  from  asphalt.  On  earth  roads  in  poor 
condition  the  wear  is  severe,  and  on  stone  blocks  it  is 
estimated  to  be  three  or  four  times  as  great  as  on 
asphalt.  The  financial  value  of  the  saving  in  this  wear 
and  tear  is  difficult  to  ascertain,  but  it  is  undoubtedly 
sufficient  to  make  it  an  important  item  in  the  cost  of 
highway  transportation. 

Land  Values. — The  effect  of  highway  improvements 
upon  the  value  of  adjoining  property  is  dependent 
upon  the  nature  of  the  uses  to  which  the  property  may 
be  put,  and  the  extent  to  which  various  characteristics 
of  the  road  surfaces,  such  as  dust  or  noise,  may  affect 
the  occupations  or  comfort  of  the  occupants.  A 
pavement  may  thus  sometimes  have  a  direct  effect 


14       A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

upon  rental  values.  In  general,  however,  the  effect  is 
difficult  to  estimate,  although  it  is  commonly  recog- 
nized in  the  practice  of  assessing  a  portion  of  the  cost 
of  improvements  against  abutting  property. 

The  value  of  comfort,  convenience,  safety,  and 
healthfulness  to  a  community,  as  affected  by  the  con- 
dition of  their  roads  and  streets,  cannot  readily  be 
stated  in  figures;  but  they  have  a  money  value,  both 
in  their  effect  upon  the  general  life  and  business  of  the 
community  and  in  the  attraction  presented  to  outside 
business  enterprises  or  home-seekers. 

ART.  6.    HEALTHFULNESS. 

The  effect  of  a  pavement  upon  the  health  of  the 
residents  of  its  locality  will  be  affected  by  the  tendency 
of  the  materials  composing  it  to  decay,  by  its  permea- 
bility, and  by  its  degree  of  freedom  from  noise  and 
dust. 

The  permeability  of  a  road  surface  is  important  on 
account  of  the  tendency  of  surface-water  and  refuse 
matter  to  penetrate  and  saturate  it,  and  thus  cause  it 
to  become  dangerous  to  health.  A  continuous  sheet 
pavement  is  the  most  desirable  in  this  particular,  and 
a  block  pavement  with  open  joints  the  least  so.  When, 
however,  the  joints  of  a  block  pavement  are  properly 
cemented,  the  pavement  may  be  made  nearly  imper- 
vious. If  the  material  of  which  the  pavement  is  com- 
posed be  permeable,  it  may  gradually  become  saturated 
with  street  refuse,  even  though  the  joints  be  made 
tight,  and  where  the  material  is  liable  to  decay  it  may 
of  itself  become  obnoxious  to  health. 

Both  these  objections  are  raised  to  the  use  of  wood 


GENERAL  CONSIDERATIONS.  15 

pavements,  and  probably  in  many  cases  with  justice. 
This  is  a  matter,  however,  concerning  which  authorities 
differ.  The  extent  of  the  danger  to  health  involved 
in  the  use  of  wood  for  pavements  in  any  particular 
case  probably  depends  largely  upon  the  wood  selected 
for  use,  and  the  method  of  construction  adopted.  It 
is  at  least  questionable  whether  the  permeability  of  the 
material  used  for  pavements  is  in  practice  ever  as  ob- 
jectionable on  the  ground  of  health  as  that  caused 
by  open-joint  construction  of  block  pavement,  even 
though  the  material  of  the  blocks  be  impervious  to 
moisture. 

The  noise  made  by  traffic  upon  a  pavement  is  impor- 
tant not  only  because  of  its  effect  upon  the  comfort  of 
the  people  using  it  or  living  adjacent  to  it,  but  also 
because  to  it  are  frequently  attributed  many  nervous 
disorders  to  which  people  in  some  cities  are  subject. 

Stone-block  pavements  are  the  most  objectionable 
in  this  particular,  causing  a  continual  roar,  due  both  to 
the  rumbling  of  wheels  over  them  and  the  blows  of  the 
horses'  feet  upon  them.  Upon  asphalt  the  noise  is  only 
that  due  to  the  horses'  feet,  giving  a  sharp,  clicking 
sound.  Upon  wood  the  horses  produce  no  appreci- 
able sound  ;  but  wheels  give  a  dull  rumble,  generally 
considered  the  least  objectionable  of  any  of  the  noises 
made  by  the  more  common  pavements.  The  noise  of 
wood  pavements  is  diminished  by  making  the  joints 
between  blocks  small.  A  brick  surface  gives  a  combi- 
nation of  the  sounds  of  wood  and  asphalt,  the  clicking 
being  much  less  sharp  than  on  asphalt,  and  the  rumble 
less  noticeable  than  on  wood.  On  any  block  pave- 
ment the  noise  is  lessened  as  the  foundation  is  made 
more  firm  and  the  joints  more  close  and  well  cemented. 


l6       A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

An  earth  or  broken-stone  road  is  usually  less  noisy 
than  any  of  the  hard  pavements. 

The  giving  off  of  dust  by  a  pavement  under  the 
action  of  traffic  is  also  objectionable  on  the  score  of 
health  as  well  as  of  comfort.  All  pavements  produce 
more  or  less  dust,  the  amount  depending  more  upon 
the  method  of  construction  and  care  used  in  forming 

o 

the  surface  and  filling  the  joints  than  upon  the  material 
of  the  pavements.  For  the  most  part,  however,  the 
presence  of  dust  is  dependent  rather  upon  the  main- 
tenance, cleaning,  and  sprinkling  of  the  pavement  than 
upon  its  nature,  and  the  dirt  upon  the  surface  of  a 
hard  pavement  is  usually  carried  there  from  the  out- 
side and  not  due  to  the  pavement. 

Earth  and  broken-stone  roads  wear  rapidly,  and 
make  dust  freely  in  dry  weather,  requiring  frequent 
sprinkling  and  cleaning  to  keep  the  road  clear  of  it, 
and  are  on  this  account  objectionable  for  use  on  the 
streets  of  towns  under  any  considerable  traffic. 

ART.  7.     SAFETY. 

The  safety  of  a  road  surface  depends  upon  the  foot- 
hold afforded  by  it  to  horses  under  normal  conditions, 
and  also  upon  the  degree  of  slipperiness  that  it  may 
take  in  wet  weather,  or  under  the  influence  of  ice  and 
snow  in  winter. 

A  dry  earth  road  in  good  condition  gives  the  best 
and  surest  foothold,  with  broken-stone  and  gravel 
roads  nearly  as  good. 

The  relative  safety  of  the  various  pavements  used  in 
city  streets  is  a  matter  upon  which  there  is  consider- 
able difference  of  opinion  amongst  authorities.  Local 


GENERAL  CONSIDERATIONS.  17 

conditions  affect  the  pavement  in  this  regard  to  an  im- 
portant degree.  The  dampness  of  the  climate,  the 
shade  from  buildings,  the  cleanliness  of  the  streets, 
and  the  prevalence  of  snow  and  ice  in  winter  are  all 
important. 

Statistics  upon  the  question  of  relative  safety  of 
wood,  asphalt,  and  granite  have  been  collected  by 
Capt.  Greene  in  this  country  and  by  Col.  Haywood 
in  London,  the  attempt  being  made  to  determine 
the  number  of  miles  travelled  by  horses  upon  each 
kind  of  pavement  to  each  accident  due  to  slipperi- 
ness. 

The  results  of  Col.  Haywood  seem  to  show  that 
of  the  three  wood  is  the  safest  and  granite  the  most 
dangerous,  while  the  results  of  Capt.  Greene  show 
asphalt  to  be  the  best  and  wood  the  worst  in  this 
particular. 

Col.  Haywood's  observations  were  all  taken  on 
London  streets,  and  are  as  follows  : 

Miles  travelled  to  each  fall  on 
Granite.          Asphalt.  Wood. 

In  dry  weather,  78  223  646 

"  damp  "  168  125  193 

"  wet  "  432  192  537 

All  observations,  132  191  330 

The  observations  were  made  when  dry  weather 
prevailed,  and  therefore  are  somewhat  unfavorable  to 
granite,  which  is  safest  when  wet. 

Capt.  Greene's  observations  were  made  in  several 
American  cities,  and  showed  the  distance  travelled  to 
each  fall  to  be,  on  granite  413  miles,  on  asphalt  583 
miles,  and  on  wood  272  miles.  The  observati< 


1 8       A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

wood  in  this  series  were  too  few  to  give  a  reliable  in- 
dication, and  it  is  to  be  observed  with  regard  to  all  of 
them  that  slipperiness  is  largely  affected  by  the  con- 
dition in  which  the  surface  is  maintained,  and  it  is 
therefore  difficult  to  draw  any  general  conclusions 
which  would  fit  all  cases. 

All  hard  pavements  are  slippery  when  muddy  and 
wet,  and  cleanliness  is  the  necessary  condition  *of 
safety. 

Wood  and  asphalt,  if  clean,  are  least  slippery  when 
dry  and  most  so  when  simply  damp.  Granite,  after 
the  surface  becomes  worn  and  polished,  is  most  slip- 
pery when  dry  and  least  so  when  wet. 

Under  a  light  fall  of  snow  both  wood  and  asphalt 
become  very  slippery,  and  in  freezing  weather  wood 
sometimes  becomes  slippery  through  the  freezing  of 
the  moisture  retained  by  it. 

No  statistics  are  available  as  to  the  safety  of  brick 
pavements,  but  it  is  thought  a  desirable  material  in 
this  respect. 

It  may  also  be  remarked,  that  the  danger  of  a  horse 
falling  upon  any  pavement  depends  very  largely  upon 
the  training  of  the  animal  and  whether  he  be  accus- 
tomed to  the  particular  surface  in  question. 

ART.  8.     DURABILITY  OF  VARIOUS  SURFACES. 

The  durability  of  a  road  or  pavemjnt  is  dependent 
upon  so  many  circumstances  connected  with  local  con- 
ditions, the  nature  of  the  traffic,  methods  of  con- 
struction, and  efficiency  of  maintenance,  that  any 
comparison  of  the  various  kinds  of  pavement  in  this 
respect  is  difficult  and  likely  to  be  misleading. 


GENERAL  CONSIDERATIONS.  ig 

The  qualities  which  especially  affect  the  durability 
of  the  road  may  be  partially  enumerated  as  follows : 

(1)  The  hardness  and  toughness  of  the  material  com- 
posing the  surface,  upon  which  depends  the  resistance 
of  the  surface  to  the  abrading  action  of  the  wheels  and 
horses'  feet  passing  over  it. 

(2)  The  firmness  of  the  foundation,  which  serves  to 
distribute  the  loads  over  the  road-bed,  and  keep  the 
surface  uniform. 

(3)  The  drainage   of  the  road-bed,  which  can  only 
properly  sustain  the  loads  which  come  upon  it  when  it 
is  dry. 

(4)  The   permeability  of  the  surface,  which   should 
form  a  water-tight   covering  to   serve  the  purpose  of 
keeping  the  foundation  and   road-bed   in  a  dry  con- 
dition. 

(5)  The  resistance  of  the  materials  of  the  pavement 
to  the  disintegrating  influences  of  the  atmosphere,  and 
to  the  action  of  the  weather. 

The  relative  importance  of  these  various  factors,  in 
any  particular  case,  depends  largely  upon  the  nature 
and  extent  of  the  traffic  which  is  to  pass  over  the 
pavement. 

The  amount  of  traffic  to  which  a  street  is  subjected 
is  usually  estimated  in  terms  of  tons  per  foot  of  width 
of  street,  by  observing  the  number  of  teams  passing  a 
given  point  during  certain  times,  classifying  them,  and 
assigning  an  average  value  of  load  to  each  class.  The 
wear  of  the  surface  will  naturally  be  somewhat  propor- 
tional to  the  amount  of  traffic.  The  life  of  a  pave- 
ment is,  however,  affected  by  other  conditions,  and 
hence  cannot  always  be  inferred  from  the  amount  of 
traffic. 


20       A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

Traffic  may  also  be  classified  according  to  its  nature 
as  heavy  or  light,  depending  upon  the  weight  of  indi- 
vidual loads  which  are  carried.  It  is  the  heavy  loads 
borne  upon  narrow  wheel-tires  that  do  the  greatest 
damage  to  a  pavement,  and  hence  the  nature  rather 
than  the  amount  of  traffic  determines  the  character  of 
pavement  necessary. 

Granite  blocks,  where  a  firm  unyielding  foundation 
is  employed,  give  the  hardest  and  most  durable  surface 
of  any  of  the  common  pavements.  This  is  especially 
the  case  under  very  heavy  loads. 

Asphalt  and  brick  rank  next  to  stone,  and  when 
well  constructed  are  satisfactory  under  any  but  the 
heaviest  traffic.  The  relative  durability  under  wear  of 
brick  and  asphalt  is  a  matter  of  doubt,  both  materials 
being  subject  to  considerable  variations  in  quality, 
and  showing  varying  results  in  different  localities,  due 
both  to  differences  in  the  quality  of  the  material  and 
in  the  methods  of  construction. 

Wood  is  less  durable  and  only  suitable  for  com- 
paratively light  traffic,  unless  its  other  advantages  be 
considered  worth  the  high  cost  of  maintenance  under 
heavier  traffic,  as  has  been  the  case  in  London,  where 
wood  has  been  largely  used  under  traffic  which  required 
its  renewal  every  four  or  five  years. 

Broken  stone  wears  rapidly  under  moderately  heavy 
traffic,  and  should  be  employed  only  on  suburban 
streets  or  country  roads  used  mainly  for  light  driving 
or  a  small  amount  of  traffic. 

The  durability  of  any  pavement  also  depends  largely 
upon  the  system  employed  for  maintaining  it,  and 
upon  its  being  kept  clean.  Cleanliness  is  specially 
important  with  wood,  asphalt,  and  broken  stone. 


GENERAL   CONSIDERATIONS. 


21 


Brick    or    stone    blocks    are  not  so  much  injured  by 
neglect. 

The  wear  of  a  pavement  also  depends  largely  upon 
the  smoothness  of  the  surface,  as  the  impacts  to  which 
the  material  is  subjected  are  produced  by  irregularities. 
So  that  the  most  durable  material  may  not  always  give 
the  greatest  resistance  to  wear. 


CHAPTER   II. 
DRAINAGE   OF   ROADS   AND   STREETS. 

ART.  9.     NECESSITY  FOR  DRAINAGE. 

THE  road-bed,  usually  formed  of  the  natural  earth 
over  which  the  road  or  pavement  is  to  be  constructed, 
must  always  carry  the  loads  which  come  upon  the 
road  surface.  Where  an  artificial  road  surface  or 
pavement  is  employed,  the  earth  road-bed  is  protected 
from  the  wear  of  the  traffic,  and  the  \vheel  loads  com- 
ing upon  the  surface  are  distributed  over  a  greater 
area  of  the  road-bed  than  if  the  loads  come  directly 
upon  the  earth  itself ;  but  the  loads  are  transferred 
through  the  pavement  to  the  road-bed,  and  not  sus- 
tained by  the  pavement  as  a  rigid  structure. 

The  ability  of  earth  to  sustain  a  load  depends  in  a 
large  measure  upon  the  amount  of  moisture  contained 
by  it.  Most  earths  form  a  good  firm  foundation  so 
long  as  they  are  kept  dry,  but  when  wet  they  lose 
their  sustaining  power,  becoming  soft  and  incoherent. 
When  softened  by  moisture  the  soil  may  be  easily 
displaced  by  the  settling  of  the  foundation  of  the 
road,  or  forced  upward  into  any  interstices  that  may 
exist  in  its  superstructure. 

In  cold  climates  the  drainage  of  a  road  is  also  im- 
portant because  of  the  danger  of  injury  from  freezing. 
Frost  has  no  disturbing  effect  upon  dry  material,  and 

22 


DRAINAGE   OF   ROADS   AND    STREETS.  23 

hence  is  an  element  of  danger  only  in  a  road  that  re- 
tains water. 

In  order,  therefore,  that  the  loads  may  be  uniformly 
sustained,  and  the  surface  of  the  road  kept  firm  and 
even,  it  is  evidently  of  first  importance  that  the  road- 
bed be  maintained  in  a  dry  condition.  This  may  be 
accomplished  by  the  use  of  an  impervious  road  cover- 
ing, by  proper  underdrainage,  or  by  a  combination  of 
the  two,  as  may  be  necessary  in  any  particular  case. 

An  impervious  surface  is  always  desirable,  not  only 
as  a  means  of  keeping  the  road-bed  dry,  but  also  as  a 
protection  to  the  pavement  itself  against  the  disin- 
tegrating action  of  water  and  of  the  weather  upon 
the  materials  of  the  surface.  Such  a  surface  is  not, 
however,  always  practicable,  and  other  means  must 
often  be  used  to  free  the  road  from  water. 

The  necessity  for  underdrainage  in  any  case  de- 
pends upon  local  conditions,  the  nature  of  the  soil,  and 
the  tendency  of  the  site  to  dampness,  as  well  as  the 
permeability  of  the  surface. 

The  object  should  be  as  far  as  possible  to  prevent 
water  from  reaching  the  road-bed,  and  to  provide 
means  for  immediately  removing  such  as  does  reach  it 
before  the  soil  becomes  saturated  and  softened. 

ART.  10.    SURFACE  DRAINAGE. 

The  drainage  of  the  surface  of  a  road  is  provided  for 
by  making  the  section  higher  in  the  middle  than  at  the 
sides,  with  ditches  or  gutters  at  the  edges  of  the  road 
along  which  the  water  is  conducted  until  it  may  be  dis- 
posed of  through  some  side  channel. 

The  slope  necessary  from  the  middle  to  the  sides  of 


24        A   TEXT-BOOK   ON   ROADS   AND    PAVEMENTS. 

the  road  to  insure  good  drainage  depends  upon  the 
nature  of  the  covering,  being  less  as  the  road  surface 
is  more  smooth  and  less  permeable  to  water.  It  varies 
from  about  I  in  20  or  I  in  30  for  broken  stone  to  I  in 
40  or  i  in  60  for  various  classes  of  pavement,  and  for 
asphalt  sometimes  as  low  as  I  in  80. 

The  form  of  section  used  is  commonly  either  a  con- 
vex curve,  approximately  circular,  or  it  is  made  up  of 
two  plane  surfaces  sloping  uniformly  from  the  middle 
to  the  sides  in  each  direction,  and  joined  in  the  middle 
by  a  small  circular  arc.  There  has  been  considerable 
dispute  among  engineers  as  to  which  of  these  forms  is 
most  desirable,  although  the  general  preference  seems 
to  be  given  the  plane  section.  It  is  not  usually  a 
matter  of  special  importance,  provided  the  section 
used  is  not  too  flat  at  the  middle  for  good  drainage, 
and  not  too  steep  at  the  gutters  for  safety.  In 
places  where  surface-water  must  be  carried  for  con- 
siderable distances  in  gutters  at  the  side  of  the  road, 
and  provision  must  be  made  for  a  considerable  flow, 
the  gutters  may  be  deepened  by  increasing  the  slope 
of  the  surface  at  the  sides,  or  rounding  off  as  much 
as  possible  without  making  the  slope  too  steep  for 
safety. 

The  road  should  also  have  a  certain  longitudinal 
slope  in  order  that  the  water  may  flow  freely  in  the 
gutters.  This  slope  should  be  at  least  I  in  200  in 
most  cases  in  paved  streets,  and  somewhat  greater — 
about  i  in  100  to  I  in  120 — on  broken-stone  or  earth 
roads.  Where  longitudinal  slopes  are  steep,  some  pro- 
vision must  be  made  to  prevent  the  wash  of  the  gut- 
ters, and  in  such  places  it  is  specially  desirable  to  take 
the  water  from  the  gutters -as  frequently  as  possible, 


DRAINAGE   OF   ROADS   AND   STREETS.  2$ 

in  order  to  make  the  gutter  flow  small.  This  may 
often  require,  where  no  sewers  exist,  the  laying  of 
a  special  pipe  underground  for  the  purpose. 

On  country  roads  the  disposal  of  surface-water  is 
not  usually  a  matter  of  difficulty,  as  it  can  be  carried 
along  the  road  and  run  into  the  first  convenient  cross- 
channel. 

In  towns  the  most  satisfactory  method  of  disposing 
of  surface  drainage  is  through  a  system  of  storm 
sewers,  the  water  collected  in  the  gutters  being  emptied 
at  frequent  intervals  into  the  sewers  and  thus  quickly 
removed  from  the  surface  of  the  street.  In  the  ab- 
sence of  such  a  system  it  may  often  be  necessary  to 
lay  pipes,  connecting  with  the  nearest  natural  channel, 
to  relieve  the  gutters.  In  such  cases  catch-basins 
should  always  be  placed  at  the  entrance  to  the  pipe 
to  prevent  it  getting  clogged  by  the  dirt  which  may 
be  washed  in  from  the  gutter.  On 
lines  of  pipe  of  considerable  length, 
catch-basins  should  also  be  intro- 
duced at  intervals,  to  allow  the 
accumulated  sediment  to  settle  and 
be  removed. 

Fig.  3  represents  a  basin  of  this 
kind.  It  may  be  formed  for  small 
pipes,  of  a  length  of  pipe  set  on 
end  with  the  lower  end  closed,  or 
where  necessary  a  box  built  of 
masonry  may  be  employed. 

In  all  cases  it  is  important  that  the  water  which 
falls  upon  the  surface  should  be  gotten  rid  of  as  soon 
as  possible,  for  so  long  as  it  remains  upon  the  road  it 
it  is  an  element  of  danger,  both  from  its  tendency  to 


FIG.  3. 


26       A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

wash  the  surface,  and  from  its  liability  to  penetrate 
into  the  road  and  thus  cause  disintegration  or  settle- 
ment. The  best  method  of  removing  this  water  in 
any  particular  case  must  be  determined  by  a  careful 
study  of  local  conditions,  and  its  final  disposal  in  the 
case  of  the  streets  of  a  town  is  a  special  problem  re- 
quiring careful  treatment. 

ART.    II.       SUBDRAINAGE. 

The  drainage  of  the  sub-soil  of  a  road-bed  may  be 
directed  either  to  the  removal  from  the  road-bed  of 
water  that  percolates  through  the  road  covering,  or  to 
the  prevention  of  sub-surface  waters  from  reaching 
and  saturating  the  road-bed. 

The  necessity  for  subdrainage,  and  the  method  to 
be  employed  in  any  case,  depends  upon  whether  the 
soil  over  which  the  road  is  being  constructed  is  natu- 
rally wet  or  dry,  and  whether  the  road-bed  is  so  situ- 
ated and  formed  as  to  give  it  natural  drainage. 

Where  artificial  subdrainage  is  necessary  the  drains 
should  be  located,  in  so  far  as  possible,  with  a  view  to 
cutting  off  the  supply  of  water  before  it  reaches  the 
road-bed.  To  accomplish  this  to  the  best  advantage 
the  local  conditions  must  be  observed,  the  sources  of 
this  supply  determined,  and  the  nature  of  the  under- 
flow, if  any  exist,  considered. 

In  many  situations,  particularly  when  the  site  of  the 
road  is  low  and  naturally  damp  in  wet  weather,  it  may 
be  advisable  to  place  a  longitudinal  drain  under  each 
side  of  the  road.  Such  a  construction  is  shown  in 
Fig.  4,  which  gives  a  section  of  a  macadamized  country 
road  with  tile  side-drains. 


DRAINAGE   OF  ROADS   AND   STREETS.  2? 

Frequently,  as  in  many  cases  of  a  road  along  a  side 
slope,  there  is  a  well-defined  flow  of  sub-surface  water 
from  one  side  to  the  other,  and  in  such  case  the  water 


FIG.  4. 

may  perhaps  be  intercepted  by  a  single  longitudinal 
drain  on  the  side  of  the  roadway  from  which  the 
water  comes.  An  example  of  this  is  shown  in  Fig.  5, 


FIG.  5. 

which   represents   a  macadamized  village   street  with 
stone  curb,  gutters,  and  sidewalks. 

In  other  cases,  where  the  subsoil  is  of  a  very  reten- 
tive nature,  or  where  the  natural  slope  of  the  land  is 
in  the  direction  of  the  length  of  the  road,  cross-drains 
leading  into  a  longitudinal  side-drain  or  into  side- 
ditches  may  be  expedient,  and  sometimes,  especially 
upon  narrow  country  roads,  a  single  longitudinal  drain 
under  the  middle  of  the  road  may  give  the  best  re- 
sults, serving  both  to  remove  sub-surface  water  and 
that  which  percolates  through  the  road  surface.  Fig. 
6  shows  such  a  road,  representing  an  ordinary  earth 
road  with  a  tile  centre-drain.  Fig.  20  also  represents 
a  stone  centre-drain  as  sometimes  used  under  a  broken- 
stone  road  over  wet  ground. 


28        A  TEXT-BOOK   ON   ROADS   AND    PAVEMENTS. 

These  longitudinal  drains  should  be  arranged  to 
empty  as  frequently  as  possible  on  country  roads  into 
the  natural  drainage-channels,  or  in  towns  into  sewers 
arranged  to  convey  the  water  rapidly  away. 

In  general,  systematic  underdrainage  will  not  be 
necessary  except  in  localities  where  the  ground  is  natu- 
rally damp  from  lack  of  natural  drainage,  or  where  an 


FIG.  6. 

underflow  creates  a  tendency  to  wetness  in  the  sub- 
soil. In  some  localities,  however,  upon  country  roads, 
where  an  impervious  surface  is  not  employed  and  the 
soil  is  one  that  absorbs  and  retains  water,  it  may  be 
necessary  to  provide  subdrainage  to  remove  water 
that  passes  through  the  road  surface.  This  is  most 
commonly  done  by  a  series  of  shallow  cross-drains  or 
by  a  single  longitudinal  one  in  the  middle  of  the  road. 
In  a  town  where  sewers  traverse  the  streets  subsoil 
drainage  is  easily  arranged  for  in  connection  with  the 
sewers.  Frequently  blind-drains  or  stone-drains  are 
constructed  underneath  or  alongside  the  pipes.  In 
other  cases  good  drainage  is  secured  by  drains  under- 
neath the  curb  or  gutter,  which  are  connected  with  the 
sewers. 

ART.  12.     KINDS  OF  SOIL. 

The  material  of  which  a  road-bed  is  composed  is 
important   because   it   determines   to  a   large   extent 


DRAINAGE   OF   ROADS  AND   STREETS.  2Q 

whether  artificial  drainage  is  necessary,  and  also  what 
method  should  be  adopted  for  securing  drainage. 

Soils  differ  in  their  power  to  resist  the  percolation 
of  water  through  them,  in  the  rapidity  and  extent  of 
their  absorption  of  water  with  which  they  come  in  con- 
tact, in  the  extent  to  which  moisture  renders  them  soft 
and  unstable,  and  in  their  power  of  retaining  moisture. 

A  light  soil  of  a  sandy  nature  usually  presents  little 
difficulty  in  the  matter  of  drainage,  as,  while  it  is  easily 
penetrated  by  water,  it  is  not  retentive  of  moisture, 
which  passes  freely  through  it  without  saturating  it 
unless  prevented  from  escaping. 

If  the  natural  drainage,  therefore,  have  a  fall  away 
from  a  road-bed  formed  of  such  material  it  will  usually 
need  no  artificial  drainage,  and  where  subdrains  are 
necessary  they  may  be  relied  upon  to  draw  the  water 
from  the  soil  to  a  considerable  distance  each  side  of 
the  drain. 

A  nearly  pure  sand  is  more  firm  and  stable,  under 
loads,  when  quite  damp  than  if  dry,  although  a  fine 
sand  saturated  by  water  which  is  unable  to  escape 
may  become  unstable  and  treacherous. 

Clays  usually  offer  considerable  resistance  to  the 
passing  of  water  through  them,  and  are  very  retentive 
of  moisture.  As  a  rule,  however,  a  clay  soil  does  not 
absorb  water  readily,  and  requires  that  water  be  held 
for  some  time  in  contact  with  it  in  order  that  it  may 
become  saturated,  although  when  saturated  it  is  the 
most  unstable  of  soils.  A  clay  that  when  dry  will 
stand  in  a  vertical  wall  and  support  a  heavy  weight 
when  wet  may  lose  all  coherence  and  become  a  fluid 
mass.  When  water  comes  in  contact  with  a  bed  of 
such  clay,  the  outside  becomes  saturated 


3O       A  TEXT-BOOK  ON    ROADS  AND   PAVEMENTS. 

fluid  before  the  moisture  penetrates  into  it  sufficiently 
to  even  moisten  it  a  few  inches  from  the  surface. 

A  clay  soil  is,  therefore,  always  difficult  to  drain  by 
removing  the  water  after  it  has  soaked  in,  or  by  per- 
mitting it  to  pass  through  the  road-bed  to  the  subdrains 
beneath.  Drainage,  in  such  cases,  may  often  be  so 
arranged  as  to  prevent  water  from  standing  against 
the  road  and  thus  prevent  it  from  becoming  saturated. 
As  the  clay  is  comparatively  non-absorptive,  the  water 
which  may  come  upon  its  surface,  if  allowed  to  escape 
at  once,  will  not  penetrate  into  it,  and  hence  will  not 
cause  softening. 

A  heavy  silt  formation  is  sometimes  met  with  which 
is  even  more  difficult  to  drain  than  a  true  clay.  It  is 
nearly  as  retentive  of  moisture  as  a  clay,  strongly  re- 
sisting the  passage  of  water  through  it,  but  at  the  same 
time  absorbs  water  quite  freely  when  in  contact  with  it. 

Between  the  extremes  mentioned  above  there  are 
a  great  number  of  varieties  of  soil  which  possess  to  a 
greater  or  less  extent  the  characteristics  of  either  or 
both,  and  gradually  merge  the  one  into  the  other.  In 
applying  a  system  of  drainage  in  any  case,  careful 
attention  should  always  be  given  to  the  characteristics 
of  the  soil,  as  determining  very  largely  the  treatment 
to  be  used. 

In  pervious,  sandy,  or  gravelly  soil  drains  may  often 
be  effective  for  a  distance  of  30  or  40  feet  through  the 
soil,  while  with  a  less  pervious  retentive  clay  the  drain 
may  not  act  effectively  more  than  8  or  10  feet  on  each 
side. 


DRAINAGE  OF  ROADS  AND   STREETS.  31 

ART.  13.    TYPES  OF  DRAINS. 

For  the  purpose  of  draining  the  subsoil  of  the  road- 
bed the  drains  used  may  be  either  open  ditches  at  the 
sides  of  the  roads  or  porous  covered  drains. 

Open  ditches  are  sometimes  used  on  country  ro?ds. 
They  are  usually  placed  at  the  extreme  edges  of  the 
road,  and  must  be  deep  in  order  to  be  effective.  Being 
so  far  from  the  travelled  portion  of  the  road,  they  can 
only  act  satisfactorily  as  subdrains  where  the  soil  is 
pervious  and  easily  drained. 

In  other  cases,  where  side-ditches  are  employed, 
covered  cross-drains  must  be  introduced  to  carry  the 
water  from  the  middle  of  the  road  to  the  ditches.  Fig. 
7  shows  a  section  of  a  country  road  drained  by  side- 


FIG.  7. 

ditches.  Where  such  ditches  are  employed  the  slope 
of  the  sides  should  be  made  as  gradual  as  possible,  at 
least  ij-  or  2  horizontal  to  I  vertical,  in  order  to  dimin- 
ish the  danger  of  the  washing  of  the  banks,  as  well  as 
the  liability  to  overturning  of  a  vehicle  over  the  edge. 
Covered  underdrains  are  to  be  preferred  to  open 
ones  for  this  use,  and  are  more  commonly  employed 
where  efficient  subdrainage  is  attempted.  These 
drains  must  be  so  arranged  that  they  may  be  read- 
ily penetrated  by  the  water  without  becoming  clogged 
by  earth  washing  into  them.  The  types  of  drains 
most  commonly  employed  for  this  purpose  are  known 
as  blind  drains,  box  or  stone  drains,  and  tile-drains. 


32       A   TEXT-BOOK  ON   ROADS   AtfD    PAVEMENTS. 

For  short  lengths,  such  as  transverse  drains  intended 
to  take  the  water  from  the  subsoil  into  the  side  ditches 
or  drains,  blind  drains  may  frequently  be  economically 
employed.  They  consist  simply  (as  shown  in  Fig.  8) 

of  ditches  cut  into  the  soil 
and  filled  with  rounded 
stones  3  to  6  inches  in  di- 
ameter. Angular  stones  are 
not  desirable  for  this  pur- 
pose, as  the  object  is  to 
leave  openings  into  which 
the  water  may  penetrate 
without  difficulty,  and  thus 
be  led  away.  The  top  of 

the  stones  must  be  covered  over  in  some  way  before 
filling  in  earth  above  in  order  to  prevent  the  earth  from 
washing  down  and  choking  the  drain.  This  is  some- 
times accomplished  by  using  smaller  stones  at  the  top, 
covered  by  a  layer  of  coarse  gravel.  A  layer  of  straw 
or  brush,  or  of  sod  turned  roots  upward  to  retain  the 
earth  until  it  becomes  thoroughly  compacted,  is  a 
common  and  effective  method  of  protecting  these 
underdrains. 

Stone-drains  are  commonly  employed  where  stone  is 
plenty  and  cheap.  They  usually  consist  of  rectangular 
or  triangular  boxes  formed  of  flat  stones  or  bricks  at 
the  bottom  of  a  trench,  which  is  then  filled  as  in  a  blind- 
drain  so  as  to  give  ready  access  for  water. 

Figs.  9  and  10  show  sections  of  stone-drains  as  com- 
monly constructed  of  rough  field-stones.  The  form 
given  in  Fig.  9  is  commonly  known  as  a  box  drain. 

Tile-drains  are  probably  in  general  the  most  con- 
venient and  efficient  for  subdrainage.  They  are  made 


DRAINAGE   OF  ROADS  AND   STREETS. 


33 


of  round,  or  U-shaped,  unglazed  drain-tile,  laid,  as  in  the 
last  case,  at  the  bottom  of  a  ditch  filled  with  round 
stones. 

Fig.  1 1  shows  the  section  of  a  tile-drain  as  commonly 
constructed. 

The  tiles  are  usually  set  end  to  end  in  the  trench, 


FIG.  9. 


FIG.  10. 


being   held   in    place   by  small  stones    braced   under- 
neath. 

The  joints  are  thus  left  open  to  permit  of  the  free 
entrance  of  water.  Collars  for  the  joints  may  be  ob- 
tained and  are  sometimes  used  ^^^^^^^^^^^^^ 
where  thought  necessary  to  keep 
larger  material  from  washing  into 
and  obstructing  the  tile.  These 
collars  are  rings  of  pipe  into  which 
the  ends  of  two  adjoining  sections 
of  the  tile  may  be  fitted,  and  they 
thus  serve  also  to  hold  the  tile  in 
line. 

The  filling  of  the  trench  above 
the  tile,  as  in  the  other  drains, 
should  be  arranged  with  a  view  to  maintaining  a 


FIG.  ii. 


34       A  TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

porous  structure  through  which  water  may  easily 
pass.  Sometimes  flat  stones  are  laid  over  the  tile 
resting  with  one  edge  on  the  bottom  of  the  trench 
and  meeting  above  at  the  middle  so  as  to  form  an 
additional  protection  to  keep  any  earthy  matter  from 
choking  the  entrance  to  the  tiles. 

All  of  these  drains  should  be  deep  enough  to  escape 
freezing. 

The  mouth  of  a  tile-drain  should  also  be  protected 
in  some  manner,  as  the  porous  tile  is  apt  to  be  broken 
and  destroyed  by  frost  when  saturated.  In  some  cases 
the  tile-drain  is  made  to  discharge  through  a  short 
length  of  stone-drain,  or  through  a  section  of  salt- 
glazed  sewer-pipe,  which  will  not  be  injured  by  freez- 
ing. 

It  is  desirable  in  all  cases  to  protect  the  mouths  of 
underdrains  with  a  netting  of  some  kind  to  prevent 
the  entrance  of  vermin,  which  may  clog  the  openings 
with  their  nests. 

The  slope  of  a  porous  drain  may  vary  from  about  I  in 
40  to  I  in  100.  In  case  a  steeper  slope  be  necessary,  a 
foundation  or  paving  should  be  placed  in  the  bottom 
of  the  trench,  which  is  otherwise  liable  to  be  eroded  by 
the  current  that  may  be  produced. 

ART.  14.    CULVERTS. 

Culverts  are  commonly  required  in  road  construction 
for  carrying  under  the  road  the  small  streams  which 
may  be  crossed  by  the  road,  or  sometimes  for  carrying 
the  water  collected  in  the  gutters  or  ditches  on  the 
upper  side  of  the  road  to  the  lower  side. 

The   waterway   provided     by   a    culvert    must,  for 


DRAINAGE   OF  ROADS   AND   STREETS.  35 

safety,  be  sufficiently  large  to  pass  the  maximum  flow 
of  water  that  is  likely  to  occur,  while  for  economy  it 
must  be  made  as  small  as  may  be  without  danger. 

The  maximum  flow  of  a  stream  depends  upon  a 
number  of  local  conditions,  most  of  which  are  very 
difficult  of  accurate  determination.  These  are  :  the 
maximum  rate  of  rainfall  ;  the  area  drained  by  the 
stream  and  its  position  ;  the  character  of  the  surface 
drained  ;  and  the  nature  of  the  channel. 

The  maximum  rate  of  rainfall  varies  in  different 
localities,  and  differs  in  the  same  locality  from  year  to 
year.  It  is  commonly  taken  at  about  an  inch  an  hour. 
This  is  sometimes  exceeded  for  a  very  short  time  and 
over  a  small  area,  but  is  usually  a  safe  value  for  a 
watershed  of  any  considerable  area. 

The  approximate  area  of  the  watershed  drained  by 
a  stream  is  readily  found,  and  its  form  is  also  impor- 
tant as  determining  the  distance  the  water  must  flow  in 
reaching  the  culvert  under  consideration,  and  to  some 
extent  regulating  the  rate  at  which  the  water  falling 
upon  the  area  will  reach  the  culvert. 

The  maximum  flow  of  a  stream  is  also  affected  by 
the  physical  characteristics  of  the  watershed.  The 
permeability  of  the  surface  largely  determines  what 
portion  of  the  rainfall  shall  reach  the  stream  ;  while  the 
slope  of  the  surface,  its  evenness,  and  its  vegetation 
have  an  effect  upon  the  quickness  and  rate  with  which 
the  rainfall  is  received  by  the  stream. 

The  determination  of  the  maximum  flow  to  be  ex- 
pected in  any  case  from  an  examination  of  the  locality 
is  therefore  possible  only  as  a  very  rough  approxima- 
tion. A  number  of  formulae  have  been  proposed  for 
such  estimation,  the  use  of  which  foriifi^cageof  an 


OF  TH 

{TJ1TI7BE 


36       A  TEXT-BOOK   ON   ROADS   AND    PAVEMENTS. 

ordinary  culvert  simply  amounts  to  estimating  the 
quantity  of  water  which  would  fall  on  the  watershed  in 
the  heaviest  probable  rain,  and  judging  as  well  as  pos- 
sible from  local  conditions  how  much  of  it  may  arrive 
at  one  time  at  the  culvert.  In  some  cases  where  a 
more  accurate  determination  is  desirable  it  may  be 
advisable  to  measure  the  flow  of  the  stream  at  high 
water,  and  form  an  idea  from  such  measurement  as  to 
what  may  be  expected  at  a  maximum  stage. 

The  amount  of  water  that  will  pass  a  culvert  in  a 
given  time  depends  upon  the  form  of  the  section,  the 
smoothness  of  its  interior  surface,  its  slope,  and  the 
head  under  which  the  water  is  forced  through.  A 
well-constructed  culvert  may  be  considered  in  comput- 
ing its  capacity  as  a  pipe  flowing  full.  Other  culverts 
or  bridges  must  be  treated  as  open  channels. 

Prof.  Talbot  gives  (Selected  Papers  C.  E.  Club,  Univ. 
of  Illinois,  1887-8)  a  formula  for  the  rough  determina- 
tion of  area  required  for  waterway,  derived  from 
experience : 


Area  waterway  in  feet  —  C  V  (drainage  area  in  acres)3. 

C  is  a  coefficient  depending  upon  local  conditions. 
For  rolling  agricultural  country  subject  to  floods  at 
time  of  melting  snow,  and  with  length  of  valley  3  or  4 
times  the  width,  C  —  -J.  When  the  valley  is  longer, 
decrease  C.  If  not  affected  by  snow  and  with  greater 
lengths,  C  may  be  taken  at  \,  \,  or  even  less.  For  steep 
side  slopes  C  should  be  increased. 

For  most  cases  in  practice  the  size  of  waterway 
required  may  be  determined  from  the  knowledge  which 
usually  exists  in  the  vicinity  regarding  the  character  of 


DRAINAGE   OF   ROADS  AND   STREETS.  37 

a  stream,  from  the  sizes  of  other  openings  upon  the 
same  stream,  or  from  comparison  with  other  streams 
of  like  character  and  extent  in  the  same  locality. 
Where  data  of  this  kind  do  not  exist,  careful  exami- 
nation of  water-marks  on  rocks,  the  presence  of  drift, 
etc.,  may  be  made  to  determine  the  height  to  which 
water  has  previously  risen. 

For  small  flow  of  water  box  culverts  of  stone  or  pipe 
culverts  are  commonly  employed.  Wooden  box  cul- 
verts are  also  sometimes  used,  constructed  of  planks  or 
heavy  timbers,  but  should  be  avoided  in  so  far  as  pos- 
sible on  account  of  their  perishable  character,  and  con- 
sequent lack  of  economy. 

Pipe  culverts  are  the  most  efficient  in  use,  and  as 
they  can  now  be  constructed  quite  cheaply  in  most 
parts  of  the  country,  are  coming  into  very  general  use. 
The  efficiency  of  a  pipe  culvert  may  frequently  be 
greatly  increased  by  arranging  it  to  discharge  under 
a  considerable  head  at  times  of  unusual  flood.  This 
requires  that  the  water  shall  freely  flow  away  below 
the  outlet,  and  that  the  surface  of  water  above  the 
culvert  may  stand  higher  than  the  head  of  the  pipe. 

Pipe  culverts  may  be  constructed  either  of  salt- 
glazed  vitrified  sewer-pipe,  or  of  iron  water-pipe. 
The  iron  pipe  possesses  greater  strength,  and  is  pref- 
erable where  a  firm  foundation  is  not  easily  obtained, 
as  it  is  not  so  easily  broken  by  any  slight  settlement. 

In  laying  pipe  culverts,  they  should  be  placed  on  a 
solid  bed,  and  the  earth  be  well  tamped  about  them. 
It  is  desirable  to  have  the  bottom  of  the  trench  exca- 
vated to  fit  the  lower  part  of  the  pipe,  depressions 
being  formed  for  the  sockets.  It  is  necessary  in  every 
case  that  the  pipe  be  firmly  and  uniformly  supported 


38       A  TEXT-BOOK   ON  ROADS  AND   PAVEMENTS. 

from  below,  in  order  that  the  culvert  may  not  be 
broken  by  settlement,  which  is  especially  likely  to 
occur  in  new  work. 

It  is  desirable  that  the  joints  in  the  pipe  be  made 
water-tight,  especially  where  the  culvert  is  likely  to  flow 
full  or  under  pressure,  as  any  water  escaping  through 
the  joints  will  tend  to  cause  a  wash  beneath  the  pipe 
and  undermine  the  culvert.  Joints  are  commonly 
filled  with  clay.  Where  strength  is  needed  the  use  of 
hydraulic  cement  mortar  is  preferable,  and  sometimes 
the  small  end  of  the  pipe  is  roughened  on  the  outside 
and  the  socket  on  the  inside  in  order  that  the  cement 
may  hold  more  firmly. 

Care  should  be  taken  that  the  culvert  have  sufficient 
slope  and  be  so  placed  that  water  may  not  stand  in 
it,  in  order  to  prevent  injury  from  freezing,  and  the 
top  of  the  culvert  pipe  should  be  at  least  two  feet 
below  the  road  surface  to  avoid  crushing. 

The  ends  of  pipe  culverts  should  be  set  in  masonry 
walls  to  give  protection  against  the  washing  of  the 
face  of  the  embankment,  hold  the  ends  firmly  in  place, 
and  prevent  the  entrance  of  water  into  the  earth  on 
the  outside  of  the  pipe. 

These  walls  to  give  efficient  protection  must  be  of 
substantial  construction,  going  down  to  a  solid  founda- 
tion below  the  bed  of  the  stream.  They  may  be  built 
of  rubble  masonry,  and  should  be  laid  up  in  hydraulic 
cement  mortar.  Such  construction  is  represented  in 
Fig.  12.  The  wall  must  extend  far  enough  on  the  side 
to  sustain  the  earth  of  the  embankment  from  the 
waterway,  or  wing  walls  may  be  used  extending  up 
stream  for  this  purpose.  The  waterway  should  be 


DRAINAGE   OF   ROADS   AND   STREETS. 


39 


paved  above  the  culvert  far  enough  to  prevent  scour- 
ing at  the  base  of  the  wall. 

For  quite  small  streams  the  walls  may  sometimes 
be  omitted  if  the  face  of  the  embankment  about  the 
entrance  to  the  pipe  and  the  waterway  for  some  dis- 
tance above  and  below  be  riprapped.  Where  it  is 
necessary  to  economize  in  the  cost  of  construction, 
this  method  is  preferable  to  the  use  of  very  light  end 
walls. 

On  streams  too  large  for  a  single  pipe  it  is  often 
economical  to  lay  two  or  three  pipes  side  by  side, 


FIG.  12. 

rather  than  to  construct  an  arch  or  the  open  way  of  a 
bridge.  In  laying  large  pipes  it  is  usually  advisable 
to  place  a  broken-stone  or  concrete  foundation  under 
the  pipes  throughout  their  lengths  to  insure  uniform 
support. 

Stone  Culverts. — Culverts  of  stone  may  be  either 
arch  culverts  or  box  culverts.  Box  culverts  are  usually 
formed  of  two  side  walls  and  a  cover.  The  side  walls 
consist  usually  of  rubble  stonework  laid  up  dry  or  in 
mortar,  as  the  case  may  be.  Where  the  stream  to  be 


40       A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 


carried  is  of  small  importance,  and  the  capacity  of  the 
culvert  not  greatly  taxed,  dry  walls  may  give  satis- 
factory results,  but  when  the  culvert  is  likely  to  flow 
full  at  certain  times  it  should  be  laid  up  in  hydraulic 
cement  mortar,  and  in  any  case  the  greater  stability 
given  by  the  mortar  would  be  well  worth  the  small 
additional  cost.  Fig.  13  shows  a  section  of  the  ordi- 


Y//////A 


W////A     V///////A 

'//////A          v/ 


FIG.  13. 

nary  form  of  box  culvert.  The  use  of  head  walls  and 
paving  the  waterway  for  a  short  distance  is  necessary 
for  these  as  for  pipe  culverts. 

Where  suitable  stone  is  available,  box  culverts  are 
easily  constructed  and  economical.  They  are  com- 
monly used  for  openings  2  to  4  feet  in  width  and 
2  to  5  feet  in  height.  The  width  that  may  be  used 
depends  upon  the  available  cover  stones.  Where  the 
allowable  width  is  not  sufficient  to  give  the  needed 
area  of  waterway,  a  double  culvert  may  sometimes  be 
used  to  advantage.  This  consists  of  two  openings 
with  a  middle  wall  to  support  the  covers. 

The  culvert's  opening  should  always  be  large  enough 
to  admit  of  a  man  passing  through  it  for  the  purpose 


DRAINAGE   OF   ROADS   AND   STREETS.  4! 

of  cleaning  it — at  least  1 8  by  24  inches.  The  side  walls 
should  extend  downward  below  the  bottom  of  the 
culvert  sufficiently  to  obtain  a  good  foundation,  and 
the  thickness  required  for  the  side  walls  usually  varies 
from  one  half  to  three  fourths  the  height,  depending 
upon  the  pressure  likely  to  come  against  them. 

In  many  cases  for  small  work  the  side  walls,  instead 
of  extending  downward,  rest  upon  the  paving  which  is 
extended  under  them.  This  gives  a  somewhat  less 
expensive  construction,  and  is  often  satisfactory  on 
good  ground. 

The  cover  stones  may  be  from  -J-  to  £  the  span  in 
thickness,  and  should  be  long  enough  to  have  a  bear- 
ing upon  each  side  wall  of  at  least  one  half  the  thick- 
ness of  the  wall. 

Arch  culverts  are  used  for  openings  too  large  to  be 
made  of  the  box  form  or  of  pipes.  The  discussion  of 
arches  and  also  that  of  bridges  in  general  is  outside 
the  proper  scope  of  this  book.  For  ordinary  country 
bridges  wooden  trusses  are  most  commonly  employed, 
on  account  of  their  comparative  cheapness.  For  short 
span  bridges  a  satisfactory  and  economical  construc- 
tion, which  has  recently  come  extensively  into  use, 
consists  in  placing  a  number  of  wrought-iron  eyebars 
across  the  opening  from  abutment  to  abutment  at 
short  distances  apart.  Brick  arches  are  then  used  to 
span  the  spaces  between  the  eyebars,  which  are  tied 
together  with  wrought-iron  rods,  and  the  roadway  is 
then  constructed  over  the  bridge  in  the  same  manner 
as  upon  the  earth  road-bed. 

Concrete  culverts  may  sometimes  be  used  to  ad- 
vantage where  they  can  be  cheaply  constructed.  They 
are  usually  made  in  oval  form,  the  bottom  being  first 
formed  by  ramming  the  concrete  upon  the  foundation 


42       A  TEXT-BOOK  ON   ROADS   AND   PAVEMENTS. 

so  as  to  form  a  curved  channel  4  to  6  inches  thick. 
The  upper  part  is  then  constructed  as  an  arch  upon  a 
centre,  which  is  left  until  the  mortar  has  set.  For  the 
small  openings  for  which  these  culvert  sare  employed 
the  thickness  of  concrete  in  the  arch  may  be  from 
£  to  £  of  the  width  of  opening.  The  concrete  for  such 
work  should  be  made  of  hydraulic  cement,  in  the 
manner  employed  in  constructing  the  foundations  for 
pavements.  (See  Art.  47.) 

Abutments  for  small  bridges  should  be  laid  upon 
solid  foundations,  and  built  of  hydraulic  cement 
mortar,  the  back  of  the  abutment  wall  being  made 
impervious  by  coating  it  with  mortar.  A  common  and 
safe  thickness  of  abutment  is  to  make  the  thickness 
•^Q  of  the  height.  The  waterway  between  abutments 
should  be  paved  to  prevent  scouring  out  the  founda- 
tions. 

ART.  15.    WATER-BREAKS. 

Upon  heavy  gradients  on  country  roads,  continuous 
for  any  considerable  distance,  water-breaks  are  com- 
monly placed  at  frequent  intervals  to  collect  the  water 
which  flows  down  the  surface  of  the  road  and  turn  it 
into  the  gutters  or  side-ditches.  They  should  only  be 
used  on  grade  steep  enough  to  make  them  essential,  as 
otherwise  they  form  an  obstruction  to  traffic.  They 
consist  of  broad  shallow  ditches,  and  should  be  arranged 
to  carry  the  water  from  the  middle  of  the  road  each 
way  to  the  gutter,  thus  forming  a  V  with  the  vertex 
uphill  and  at  the  middle  of  the  road.  This  arrange- 
ment permits  teams  following  the  middle  of  the  road 
to  cross  the  ditch  squarely.  It  is  desirable  that  these 
cross  gutters  be  paved  to  prevent  washing  during 
heavy  showers. 


CHAPTER   III. 
LOCATION   OF   COUNTRY   ROADS. 

ART.  1 6.    CONSIDERATIONS  GOVERNING  LOCATION. 

THE  determination  of  a  line  for  a  proposed  road 
involves  the  examination  of  the  country  through  which 
the  road  is  to  pass  with  reference  to  its  topographical 
features,  the  nature  and  extent  of  the  traffic  that  it 
may  develop,  and  the  local  interests  that  may  be 
affected  by  the  position  of  the  road. 

The  simplest  form  that  this  problem  can  take  is  that 
in  which  two  points,  as  two  towns,  are  to  be  connected 
by  a  road  for  the  purpose  of  providing  for  a  traffic 
between  them,  the  nature  and  amount  of  which  is 
approximately  known.  In  this  case  it  is  only  neces- 
sary to  examine  the  topography  of  the  intervening 
country  and  select  the  line  over  which,  taking  into 
account  the  costs  of  construction  and  maintenance, 
the  given  traffic  may  be  most  economically  carried. 

In  most  cases  in  practice,  however,  the  problem 
does  not  have  this  simple  character,  and  in  fact  loca- 
tion can  seldom  be  determined  by  considerations  of 
economy  alone.  The  position  of  the  line  will  be  modi- 
fied by  local  needs,  such  as  the  necessity  of  providing 
for  the  traffic  of  villages  or  farms  intermediate  between 
the  ends  of  the  road,  which  may  often  cause  deviations 

43 


44        A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

from  what  would  be  the  best  line  if  the  interests  of 
the  terminal  points  alone  were  considered. 

Questions  of  the  desirability  of  various  lines  for  the 
comfort  and  convenience  of  travel,  and  the  pleasure  to 
be  derived  from  the  use  of  the  road,  dependent  upon 
aesthetic  considerations,  may  also  frequently  operate 
to  change  the  line  from  what  would  seem  proper  from 
a  strictly  economic  point  of  view. 

In  thickly  settled  communities,  as  in  most  parts  of 
the  United  States,  the  roads  are  in  the  main  already 
located,  the  necessity  for  the  location  of  new  ones 
does  not  often  arise,  and  when  it  does  occur  is  usually 
mainly  determined  by  the  local  needs  and  requirements 
of  traffic. 

The  economic  considerations  involved  in  the  location 
of  roads  are  of  two  kinds  :  those  relating  to  the  accom- 
modation of  traffic,  and  those  relating  to  its  economic 
conduct.  The  first  deals  with  the  necessity  of  the  road 
to  the  community,  the  second  with  the  cost  of  operat- 
ing it.  The  first  involves  the  general  question  of  the 
advisability  of  any  road,  and  how  it  can  be  placed  to 
give  the  greatest  freedom  to  the  movement  of  travel. 
The  question  is  as  to  the  value  of  the  road  to  the  gen- 
eral community  and  its  location  to  secure  the  greatest 
good  for  the  least  outlay,  without  taking  into  account 
the  details  of  location  which  may  affect  the  cost  of 
transportation.  The  value  of  the  road  as  developing 
trade  in  a  town  or  bringing  a  farm  nearer  to  market 
would  enter  into  consideration.  The  accommodation 
of  traffic  requires  that  a  road  be  located  with  a  view  to 
the  convenience  of  its  use  by  the  largest  portion  of  the 
traffic,  as  well  as  with  a  view  of  developing  traffic. 

The  position  of  a  road  that  will  best  accommodate 


LOCATION  OF   COUNTRY   ROADS.  45 

traffic  is  that  in  whichj  other  things  being  equal,  the 
mass  of  traffic  need  be  moved  the  least  distance  in 
reaching  its  destination  ;  or,  in  other  words,  that  for 
which  if  each  ton  of  freight  be  multiplied  by  the  dis- 
tance through  which  it  must  be  moved  the  summation 
of  the  resulting  products  will  be  a  minimum.  If  there 
be  differences  in  the  nature  of  the  routes  over  which 
the  road  may  be  constructed,  they  may  be  considered 
as  equivalent  to  changes  in  the  relative  effective  lengths 
of  line  for  purposes  of  comparison. 

The  ordinary  problem  of  location  deals  mainly  with 
considerations  of  the  second  class.  It  consists  for  the 
most  part  in  the  relocation  of  portions  of  old  roads, 
of  making  such  changes  in  position  when  improving  a 
road  as  may  tend  to  reduce  the  cost  of  conducting 
traffic  over  it,  and  render  it  more  convenient  and 
pleasant  for  the  use  of  travel,  or  of  determining  the 
details  of  alignment  and  grade  upon  a  new  road  which 
is  approximately  fixed  in  position  by  the  purpose  of 
its  construction. 

The  most  economical  location  is  that  for  which  the 
sum  of  the  annual  costs  of  transportation,  the  annual 
costs  for  maintenance,  and  the  interest  on  the  cost  of 
construction  is  a  minimum. 

The  cost  of  conducting  transportation  is  affected  by 
the  rate  of  grade  of  the  road,  the  amount  of  rise  and 
fall  in  it,  and  the  length  of  the  road.  The  rate  of 
grade  is  important,  because  it  limits  the  loads  that  can 
be  hauled  over  the  road,  or  determines  the  number  of 
loads  that  must  be  made  to  transport  a  given  weight 
of  freight,  as  well  as  fixes  a  limit  to  the  speed  of  travel. 
The  amount  of  rise  and  fall  affects  the  expenditure  of 
power  required  to  haul  a  load  over  the  road.  The 


46        A  TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

length  of  the  road  has  an  effect  upon  the  amount  of 
work  necessary  to  haul  a  load  over  it,  the  time  required 
for  a  trip,  and  the  cost  of  maintaining  the  road  surface ; 
each  of  which,  other  conditions  being  the  same,  is 
directly  proportional  to  the  length. 

The  cost  of  construction  depends  upon  the  accuracy 
with  which  the  line  of  the  road  is  fitted  to  the  surface 
of  the  ground,  as  determining  the  amount  of  earth- 
work and  cost  of  bridges  and  culverts  ;  upon  the 
character  of  the  ground  over  which  the  road  is  to  be 
built,  which  affects  the  cost  of  executing  the  work  and 
determines  the  necessity  for  and  expense  of  drainage  ; 
and  upon  the  cost  of  land  for  right  of  way.  All  of  these 
items  must  be  considered  in  any  comparison  of  the 
cost  of  constructing  on  various  routes.  Special  care 
should  be  taken  in  selecting  a  line  to  avoid  bad  ground, 
such  as  swamps,  upon  which  construction  may  be  diffi- 
cult and  expensive.  The  availability  near  the  line  of 
the  road  of  materials  needed  for  surfacing  may  also 
become  a  matter  of  importance  in  the  cost  of  construc- 
tion, and  have  an  influence  in  determining  location. 

The  relative  importance  of  the  various  elements  af- 
fecting the  choice  of  a  line  depends  upon  the  nature 
and  amount  of  the  traffic  to  be  provided  for,  and  upon 
the  character  of  the  road  surface  to  be  used.  Where 
the  traffic  is  heavy,  the  importance  of  reducing  the 
cost  of  moving  it  by  lessening  grades  and  distance  will 
be  greater  than  where  the  traffic  is  light,  and  the  cost 
of  construction  may  be  correspondingly  increased  for 
that  purpose.  If  a  smooth  surface  be  employed,  upon 
which  traction  is  light,  the  value  of  reducing  grades 
will  be  greater  and  the  value  of  reducing  distance  less 
than  with  a  surface  of  poorer  tractive  qualities. 


LOCATION  OF  COUNTRY  ROADS.      47 

ART.  17.  LENGTH  OF  ROAD. 

Changes  in  the  length  of  a  road  affect  all  portions  of 
the  traffic  in  the  same  manner,  and  the  expenditure  of 
power  and  loss  or  gain  in  time  occasioned  by  them  are 
in  general  directly  proportional  to  their  amounts. 

The  value  of  any  considerable  saving  in  length  may 
usually  be  considered  as  equal  to  the  same  percentage 
of  the  whole  cost  of  conducting  the  traffic  that  the 
saving  in  distance  is  of  the  whole  length.  If,  therefore, 
a  rough  estimate  may  be  made  of  the  annual  traffic  to 
be  expected  upon  a  given  line  of  road  and  of  the  cost 
of  carrying  the  traffic,  this  cost  divided  by  the  length 
in  miles  through  which  the  traffic  is  moved  will  give 
+»he  annual  interest  upon  the  sum  that  may  reasonably 
be  expended  in  shortening  the  road  one  mile,  or  upon 
the  value  of  a  saving  of  a  mile  of  distance;  or  di- 
viding by  the  number  of  feet  of  distance  will  give  the 
value  of  saving  one  foot. 

It  is  to  be  noted,  however,  that  the  cost  of  the  work 
of  transportation  is  not  necessarily  proportional  to  the 
amount  of  wrork  done,  and  consequently  this  method 
would  not  be  strictly  accurate  even  were  the  data  as  to 
traffic  and  costs  readily  obtainable.  An  estimate  of 
this  character  at  best  amounts  to  only  a  rough  guess, 
but  it  may  often  be  of  use  as  an  aid  to  the  judgment 
in  deciding  upon  the  value  of  a  proposed  improvement 
involving  a  considerable  change  of  length  in  a  road. 

Where  the  road  is  so  situated  and  the  saving  in 
distance  proposed  is  such  that  it  would  enable  teams 
to  make  an  additional  trip  per  day  in  the  hauling  of 
freight,  the  difference  in  cost  of  transportation  is  quite 
tangible  and  readily  estimated  ;  but  where 


48       A  TEXT-BOOK   ON    ROADS   AND   PAVEMENTS. 

of  a  more  indefinite  nature,  or  the  saving  proposed 
insufficient  to  admit  of  additional  trips,  the  value  of 
the  difference  of  length  depends  upon  the  value  to 
other  work  of  the  small  portions  of  time  of  men  and 
teams  which  may  be  saved  by  the  shorter  route — a 
value  which  exists,  but  is  difficult  to  estimate. 

There  is  also  a  value  in  the  saving  of  distance  due 
to  the  advantage  to  the  community  of  bringing  the 
various  points  closer  together,  such  as  bringing  two 
towns  into  closer  relations  or  bringing  country  property 
nearer  to  markets.  The  method  of  considering  the 
cost  as  proportional  to  the  work  done  will  therefore 
probably  give  a  fair  idea  of  the  actual  economy  in  any 
saving  in  the  work  of  transportation. 

The  value  of  reducing  distance  varies  with  the 
character  of  the  road  surface.  As  the  cost  of  transpor- 
tation is  less  over  a  smooth  than  over  a  rough  surface, 
on  account  of  the  lighter  traction,  the  value  of  reduc- 
ing distance  is  also  less  on  the  smooth  surface. 

The  value  of  saving  distance  also  is  greater  on  a 
road  where  the  ruling  gradients  are  steep  than  upon 
one  with  light  gradients,  because  of  the  greater  num- 
ber of  loads  necessary  to  move  the  same  traffic. 

The  cost  of  maintenance  of  a  road  varies  with  its 
length,  and  under  similar  conditions  may  be  con- 
sidered, like  the  costs  of  transportation,  to  be  directly 
proportional  to  the  length  of  road. 

The  saving  in  cost  of  maintenance  from  decreasing 
distance  must  of  course  be  added  to  that  in  cost  of 
transportation  in  order  to  find  the  actual  value  of  a 
change  of  length. 

The  value  of  straightness  for  a  country  road  is  fre- 
quently very  much  overrated.  Considerable  devia- 


LOCATION  OF  COUNTRY   ROADS.  49 

tions  from  the  straight  line  may  often  be  made  with 
but  slight  increase  in  length,  and  there  seems  to  be  no 
good  reason  for  insisting  upon  absolute  straightness. 
The  error  is  commonly  made  of  sacrificing  grade  and 
expense  in  construction  to  the  idea  of  straightness 
without  the  attainment  of  any  considerable  saving  in 
length. 

It  involves  in  many  cases  the  injury  of  the  beauty  of 
the  road  and  of  the  landscape,  with  no  compensating 
economic  advantages. 

ART.  1 8.     RISE  AND  FALL. 

By  the  amount  of  rise  and  fall  is  meant  the  total 
vertical  height  through  which  a  load  must  be  lifted  in 
passing  in  each  direction  over  the  road.  It  is  distinct 
from  and  independent  of  the  rate  of  gradient. 

The  minimum  amount  of  rise  and  fall  is  found 
where  the  rise  is  all  in  one  direction  and  the  fall  in 
the  other,  each  being  equal  to  the  difference  of  eleva- 
tion of  the  terminal  points.  Any  increase  in  the  rise 
and  fall  beyond  this  amount  is  represented  by  the  rise 
encountered  in  passing  from  the  higher  to  the  lower 
terminus.  It  affects  the  traffic  equally  in  each  direc- 
tion, and  requires  a  certain  expenditure  of  power  to 
lift  the  traffic  through  the  given  rise  in  each  direction. 

If  the  cost  of  developing  the  work  necessary  to 
overcome  rise  and  fall  be  the  same  as  that  of  develop- 
ing an  equal  amount  of  work  to  overcome  distance,  the 
rise  and  fall  may  be  evaluated  in  terms  of  distance, 
and  any  change  in  rise  and  fall  may  be  considered  as 
though  it  were  a  difference  in  distance  and  treated  as 
in  Art.  17. 


50       A  TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

Where  the  rate  of  grade  is  less  than  the  angle  of 
repose  of  the  wheels  upon  the  road  surface  (see  Art. 
2)  the  work  necessary  to  overcome  rise  and  fall  will 
be  that  which  will  lift  the  load  through  a  vertical  height 
equal  to  the  amount  of  rise  to  be  considered.  When 
the  rate  of  grade  is  greater  than  the  angle  of  repose,  an 
additional  amount  of  work  must  be  done  in  applying  a 
resistance  to  prevent  the  too  rapid  descent  of  the  vehicle 
in  going  down  the  grade.  The  amount  of  this  work  in 
any  case  equals  the  work  done  in  lifting  the  load  to  a 
height  equal  to  the  difference  between  the  actual  rise 
of  the  grade  in  question  and  the  rise  of  a  grade  of  the 
same  length  and  a  rate  equal  to  the  angle  of  repose. 
Thus  on  an  ordinary  earth  road  whose  resistance  to  trac- 
tion where  level  is  100  pounds  per  ton,  suppose  a  grade 
to  occur  of  8  feet  per  100,  1000  feet  in  length.  For 
the  road  surface  we  have  100  -f-  2000  ==  .05,  and  the 
angle  of  repose  is  a  5$  grade.  Then  8$  —  5$  =  3$,  or 
the  brake-power  necessary  to  secure  uniform  motion, 
is  the  same  as  would  be  necessary  to  haul  the  load 
up  a  3$  grade,  and  a  grade  of  3  in  100  for  1000  feet 
gives  30  feet.  The  work  to  be  done  in  holding  back 
the  load  for  the  looo-ft.  grade  is  therefore  the  same  as 
would  lift  the  load  through  a  vertical  height  of  30  feet, 
or  the  fall  of  8  feet  per  100  for  1000  feet  has  the  same 
effect  as  30  feet  of  rise  in  the  same  direction,  pro- 
vided brake-power  costs  the  same  as  animal  power. 

The  value  of  rise  and  fall  in  terms  of  distance  will  de- 
pend upon  the  nature  of  the  road  surface,  as  the  work 
necessary  to  lift  a  given  load  to  a  given  height  is  a  con- 
stant, while  the  work  done  in  hauling  a  load  over  a  given 
distance  will  vary  with  the  resistance  offered  to  traction 
by  the  surface.  Thus,  taking  the  surface  as  above,  the 


LOCATION   OF  COUNTRY   ROADS.  51 

work  of  lifting  one  ton  through  a  rise  of  I  foot  is  2000 
foot-pounds,  while  with  a  tractive  force  of  100  pounds 
per  ton  2000  -=-  100  =  20  feet,  the  distance  a  ton  may 
be  moved  on  the  level  surface  in  developing  2000  foot- 
pounds of  work.  Therefore  I  foot  of  rise  or  fall  may 
be  considered  as  equivalent  to  20  feet  of  level  distance, 
and  the  value  of  reducing  the  amount  of  rise  and  fall 
may  be  found  from  that  for  reducing  distance. 

If  the  road  considered  were  a  first-class  Macadam 
road,  with  resistance  of  40  pounds  per  ton,  I  foot  of 
rise  or  fall  would  equal  2000  -f-  40  =  50  feet  of  distance. 

ART.  19.    RATE  OF  GRADE. 

The  effect  of  any  change  in  the  ruling  gradient  upon 
a  road  depends  to  a  considerable  extent  upon  what 
portion  of  the  traffic  may  be  carried  in  full  loads.  The 
lighter  portions  of  the  traffic  are  not  so  seriously 
affected  by  heavy  gradients  as  the  heavy  portions, 
although  there  is  an  advantage  in  light  gradients  for 
any  driving.  The  rate  of  speed  which  may  be  em- 
ployed will  be  less  upon  the  portions  of  the  road  having 
heavy  grade,  and  the  time  occupied  in  a  trip  over  the 
road  is  therefore  affected  somewhat  by  the  rate  of 
grade. 

The  desirability  of  a  road  for  general  driving  is 
also  much  influenced  by  the  gradients  employed,  as  is 
that  value  of  the  road  which  has  for  a  basis  the  effect 
it  may  exert  upon  the  attractiveness  of  the  locality. 
These  things  all  have  a  certain  financial  value,  which 
of  course  it  is  quite  impossible  to  estimate  with  any 
degree  of  accuracy,  but  which  should  be  considered  in 
determining  the  allowable  maximum  gradient  in  any 
case  in  practice. 


52        A  TEXT-BOOK   ON    ROADS   AND    PAVEMENTS. 

For  heavy  traffic,  such  as  the  transfer  of  goods  from 
one  town  to  another  or  the  marketing  of  country  prod- 
uce, the  limitation  of  load  placed  upon  the  traffic  by 
the  gradient  is  a  matter  of  importance,  the  effect  of 
which  is  calculable  upon  the  cost  of  transportation. 
If  in  any  case  the  approximate  amount  of  this  heavy 
traffic  which  is  likely  to  be  carried  in  full  loads  be  de- 
termined, the  relative  costs  of  its  transportation  over 
two  lines  of  differing  gradient,  other  conditions  being 
similar,  will  be  nearly  proportional  to  the  number  of 
loads  required  to  move  the  traffic  over  each  gradient. 

In  estimating  the  value  of  reducing  the  rate  of  grade, 
it  may  be  considered,  as  in  the  case  of  a  reduction  of 
length,  that  its  value  to  the  community  is  represented 
by  the  saving  in  annual  costs  of  transportation,  and 
that  the  amount  that  may  reasonably  be  expended  in 
increased  cost  of  construction  to  effect  a  reduction  of 
gradient  is  the  sum  upon  which  this  annual  saving  is 
the  interest. 

The  length  of  a  road  and  the  amount  of  rise  and 
fall  on  it  determine  the  amount  of  work  that  must  be 
done  in  hauling  a  load  over  the  road.  The  rate  of 
gradient,  on  the  contrary,  does  not  affect  the  amount 
of  work  necessary  to  move  the  traffic,  but  it  limits  the 
work  that  a  horse  may  do  at  one  trip. 

The  establishment  of  a  proper  rate  for  the  ruling 
grade  of  the  line  is,  therefore,  usually  the  most  im- 
portant point  in  location.  In  localities  where  light 
gradients  are  easily  obtained  the  problem  of  location 
is  greatly  simplified. 

By  referring  to  Article  3  the  comparative  loads  that 
a  horse  may  draw  up  different  grades  will  give  some 
idea  of  the  importance  of  carefully  considering  the 


LOCATION  OF  COUNTRY   ROADS.  53 

question  of  gradient.  In  nearly  all  cases  in  practice 
there  is  a  considerable  latitude  within  which  gradients 
may  be  chosen.  It  is  usually  a  question  of  heavier 
gradients  as  against  greater  distance  and  larger  first  cost 
for  the  road.  It  may  be  remarked  that  it  is  only  under 
exceptional  circumstances  that  it  is  either  necessary 
or  advisable  to  use  a  steeper  gradient  than  5#  on 
the  new  location  of  a  country  road  of  any  importance. 
Grades  steeper  than  the  ruling  gradient  may  some- 
times be  introduced  over  short  distances  without 
impairing  the  efficiency  of  the  road,  as  horses  are  usually 
able  to  exert  for  a  short  time  a  force  much  greater  than 
they  can  continuously  exert.  If  the  length  of  grade  be 
quite  short,  200  or  300  feet,  a  horse  can  about  double 
his  ordinary  power  in  passing  it. 

Where  long  steep  grades  must  be  used,  it  is  desirable 
to  break  them  by  short  stretches  of  lighter  gradients  to 
provide  resting-places  for  horses. 

Heavy  gradients  also  have  the  disadvantage  of  retard- 
ing traffic  in  the  direction  of  falling  grade,  and,  as 
suggested  in  Art.  18,  of  requiring  the  expenditure  of 
work  to  hold  the  load  from  too  rapid  descent. 

ART.   20.     EXAMINATION  OF  COUNTRY. 

For  the  purpose  of  obtaining  the  requisite  data 
upon  which  to  base  the  location  of  a  road,  it  is  neces- 
sary that  a  careful  examination  be  made  of  the  topo- 
graphical features  of  the  country  through  which  the 
line  is  to  pass.  The  relative  elevations  of  the  termini 
of  the  line  and  of  intermediate  points  should  be 
obtained,  and  the  directions  and  steepnesses  of  the 
various  natural  slopes  determined. 


54        A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

If  a  line  were  to  be  located  connecting  points  at  long 
distances  from  each  other,  as  sometimes  occurs  in 
railway  location,  it  would  be  necessary  to  study  the 
general  configuration  of  the  country,  noticing  the  di- 
rection of  flow  of  the  streams,  and  the  location  and 
elevations  of  the  various  passes  in  the  ridges  through 
which  it  might  be  possible  to  carry  the  line.  Usually, 
it  would  be  found  that  the  country  is  composed  of  a 
series  of  valleys,  separated  by  ridges,  branching  in  a 
systematic  manner  from  the  main  watercourse  of 
the  region,  and  that  the  passes  in  the  ridges  occur  at 
the  head  of  side  streams,  and  especially  where  streams 
flowing  into  valleys  on  opposite  sides  of  the  ridge  have 
their  sources  near  each  other. 

In  the  location  of  common  roads,  however,  the  prob- 
lem is  ordinarily  of  a  less  extended  nature,  and  may 
consist  in  joining  two  points  lying  in  the  same  valley, 
or  in  joining  points  in  adjacent  valleys  by  a  line  pass- 
ing over  a  ridge.  In  these  cases  it  is  only  necessary  to 
take  into  account  the  slope  of  the  valleys  in  question, 
the  positions  and  elevations  of  available  passes,  and  the 
side  slope  of  the  ridges. 

The  slope  of  the  bed  of  a  valley,  in  hilly  country, 
usually  forms  a  concave  curve,  the  rate  of  slope  gradu- 
ally increasing  from  the  lower  to  the  upper  end.  In  a 
valley  of  considerable  length  this  increase  in  the  rate 
of  slope  may  be  very  gradual  or,  in  short  valleys  rising 
to  a  considerable  height,  it  may  be  more  sudden.  The 
profile  ABCD  in  Fig.  14  shows  the  slope  of  a  short 
valley  which  decreases  in  slope  from  about  ten  feet 
per  hundred  at  the  upper  end  to  about  two  feet  per 
hundred  at  the  lower  end. 

When  a  map  of  the  country  to  be  traversed  is  avail- 


LOCATION   OF  COUNTRY    ROADS.  55 

able,  showing  the  positions  and  elevations  of  the  points 
controlling  the  location,  the  work  is  very  much  simpli- 
fied, the  reconnaissance  may  for  the  most  part  be 
limited  to  a  study  of  the  map,  and  the  routes  may  be 
sketched  upon  the  map  to  be  tried  in  the  field.  If  the 
map  at  hand  is  an  accurate  contour  map  on  a  sufficiently 
large  scale,  the  entire  location  may  be  worked  out  in 
detail  upon  the  map,  leaving  only  the  work  of  staking 
out  the  line  to  be  done  upon  the  ground. 

Maps  may  be  obtained,  in  most  parts  of  this  country, 
upon  which  the  horizontal  positions  of  points  may  be 
readily  fixed  with  sufficient  accuracy  for  the  purposes 
of  the  preliminary  examination.  Where  such  maps  are 
not  obtainable,  the  positions  of  points  must  be  ascer- 
tained and  a  rough  map  prepared.  For  this  purpose 
directions  may  be  measured  with  a  pocket  compass, 
and  distances  estimated  or  obtained  by  the  use  of  an 
odometer  or  pedometer,  as  may  be  most  convenient. 

Differences  of  elevation  are  easily  obtained  with  a 
fair  degree  of  accuracy  by  the  use  of  an  aneroid  ba- 
rometer, and  slopes  may  be  measured  with  a  hand 
level. 

Where  the  rough  means  ordinarily  employed  in  the 
reconnaissance  are  not  sufficiently  accurate  to  deter- 
mine the  controlling  points  of  the  lines  to  be  adopted, 
a  more  complete  examination  of  the  country  may  often 
be  made  by  a  rapid  topographical  survey  by  means  of 
the  transit  and  stadia  method. 

Whatever  means  may  be  adopted  for  doing  the 
work,  the  preliminary  examination  should  determine  a 
map  showing  the  approximate  positions  of  the  con- 
trolling points  through  which  the  road  must  pass,  and 


56        A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

enable  a  rough  sketch  to  be  made  of  the  slopes  of  the 
country  through  which  the  line  is  to  be  run. 

ART.  21.     PLACING  THE  LINE. 

After  the  preliminary  examination  of  the  locality  is 
complete  and  the  positions  and  elevations  of  the  con- 
trolling points  of  the  line  are  known  with  reference  to 
each  other,  the  line  must  be  selected  and  run  in  upon 
the  ground,  or,  if  the  reconnaissance  is  not  conclusive 
as  to  the  position  of  the  best  line,  it  is  advisable  to 
run  in  two  or  more  lines  and  make  a  more  detailed 
comparison  between  them. 

The  controlling  points  of  a  line  are  those  points  at 
which  the  position  of  the  road  is  restricted  within 
narrow  limits,  and  is  not  subject  to  change.  These 
may  be  points  where  the  location  is  governed  by  the 
necessity  of  providing  for  traffic,  or  points  where  the 
position  of  the  line  is  restricted  by  topographical  con- 
siderations, such  as  a  summit  over  which  the  line  is  to 
pass  a  ridge  or  a  favorable  location  for  a  bridge. 

Where  the  line  is  to  be  located  to  an  uniform  gradi- 
ent, it  should  be  started  from  the  controlling  point  at 
the  end  of  the  grade,  which  is  usually  the  summit.  It 
is  then  laid  off  along  the  slope  in  such  manner  as  to 
cause  it  to  have  continuously  the  rate  of  grade  decided 
upon.  Taking  D  (Fig.  14)  at  the  summit  of  the  valley 
as  the  controlling  point,  it  is  seen  that  the  distance 
from  C  to  D  is  sufficient  to  give  a  gradient  of  10  in 
100  by  following  directly  down  the  valley,  and  the  line 
with  that  gradient  may  be  run  in  that  manner. 

The  maximum  gradient  from  A  to  C  is,  however, 
only  5  in  100,  and  if  thought  advisable  the  same  maxi- 


58        A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

mum  gradient  may  be  used  between  C  and  D  by  run- 
ning the  line  DHC  diagonally  down  the  slope,  as 
shown.  This  line,  having  one  half  the  gradient,  will 
have  about  twice  the  length  of  the  line  CD. 

In  running  this  line  it  is  started  from  the  highest 
point  of  maximum  grade,  and  points  at  the  surface  of 
the  ground  are  continually  selected,  in  advance  of  the 
placing  of  the  line,  which  are  at  the  proper  elevation 
to  permit  the  grade  to  pass  through  them.  This  may 
be  accomplished  by  setting  off  the  angle  of  the  gradi- 
ent upon  the  vertical  circle  of  the  transit,  or  upon  a 
gradienter,  and  sighting  upon  a  rod  which  is  moved 
until  the  line  of  sight  strikes  it  at  the  same  height  from 
the  ground  that  the  instrument  is  setting.  The  points 
for  the  line  may  also  be  found  by  running  a  line  of 
levels  ahead  of  the  transit  line  (a  hand  level  is  conveni- 
ent for  this  purpose)  and  pacing  distances  upon  which 
to  reckon  the  gradient,  the  distances  and  elevations 
being  frequently  checked  upon  those  of  the  measured 
line. 

The  location  of  a  gradient  upon  a  common  road 
differs  from  that  upon  a  railroad  only  in  that  steeper 
gradients  are  used,  sharper  curves  or  angles  may 
be  employed,  and  the  gradients  need  not  be  lessened 
on  ordinary  bends  or  curves.  If  the  line  is  to  make 
a  turn  upon  the  slope  as  at  H,  the  grade  should  be 
flattened  at  the  turn,  and  a  curve  of  as  large  radius  as 
possible,  without  too  great  expense  for  grading,  be  in- 
troduced. 

In  a  manner  similar  to  the  above  a  line  might  be 
run  from  D  on  the  other  side  of  the  valley,  which 
using  a  5$  gradient  would  give  the  line  DML,  reach- 
ing the  bed  of  the  valley  at  the  point  L<  A  lighter 


LOCATION   OF  COUNTRY   ROADS.  59 

gradient  may  be  obtained  from  A  to  D  by  starting 
from  D  and  going  down  by  a  continuous  gradient  of 
4  in  100  on  the  line  D  F  G  A,  and  greater  or  less  rates 
of  descent  may  be  adopted  and  lines  corresponding  to 
them  located,  as  may  be  considered  advisable. 

The  centre-line  for  a  final  location  should  be  care- 
fully run,  and  points  permanently  marked  from  which 
it  may  be  relocated  when  necessary.  An  accurate  line 
of  levels  should  also  be  run  over  the  centre-line  and  a 
profile  drawn,  upon  which  the  grades  may  be  estab- 
lished and  earthwork  estimated. 

After  placing  the  centre-line,  topography  should  be 
taken  carefully  upon  each  side  of  the  line  for  some 
distance,  and  a  map  drawn  showing  the  topography 
and  giving  elevations  by  means  of  contours.  This  will 
serve  to  show  whether  the  line  is  placed  to  the  best 
advantage,  and  whether  any  changes  are  desirable. 
This  is  especially  necessary  over  rough  ground  or 
where  the  line  is  on  maximum  gradient,  as  frequently, 
and  perhaps  usually,  the  first  line  run  will  be  useful 
only  as  a  preliminary  line,  which  with  its  accompany- 
ing topography  will  permit  a  proper  location  to  be 
made. 

ART.  22.    COMPARISON  OF  ROUTES. 

In  selecting  a  line  for  the  construction  of  a  road  the 
principles  already  mentioned  in  the  early  part  of  this 
chapter  should  be  had  in  mind.  The  line  must  be  well 
designed  to  accommodate  the  traffic.  It  should  have 
as  easy  grades,  short  length,  and  small  rise  and  fall  as 
is  consistent  with  a  reasonable  cost  of  construction,  in 
order  to  give  light  costs  for  transportation  and  for 
maintenance. 


60        A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

Suppose  in  the  case  shown  in  Fig.  14  that  it  is  desired 
to  connect  the  village  at  the  point  A  with  the  point  D 
and  with  the  roads  leading  through  the  passes  at  F 
and  /.  Which  line  it  will  be  the  most  advantageous 
to  adopt  depends  upon  the  relative  importance  of  the 
traffic  to  the  various  points  considered. 

The  shortest,  and  probably  cheapest,  line  from  A 
to  D  would  be  obtained  by  following  the  valley  over 
the  line  ABCD,  which  line,  as  shown  by  the  profile, 
would  give  a  maximum  gradient  of  10  in  ico  between 
C  and  D.  The  line  FB  joining  the  first  line  at  B 
would  afford  communication  with  the  summit  at  F 
with  a  maximum  gradient  of  5  in  100.  If  the  traffic  to 
the  point  D  be  small  and  unimportant,  so  that  addi- 
tional expense  in  reducing  the  gradient  from  C  to  D 
is  unadvisable,  these  lines  might  prove  a  satisfactory 
location. 

If,  however,  D  be  a  point  of  importance  and  the 
traffic  from  A  toD  heavy,  it  will  be  necessary  to  adopt 
some  means  to  reduce  the  gradient  from  C  to  D. 
Leaving  out  of  consideration  the  point  F  and  consider- 
ing B  and  C  as  points  of  minor  importance,  it  might 
be  advisable  to  use  the  line  ALMD  with  an  uniform 
5$  gradient  from  D  to  L,  and  branches  to  connect 
with  C  and  B.  This  would  give  a  line  but  little  longer 
than  the  valley  line,  with  only  one  half  the  ruling  gra- 
dient of  that  line. 

If  C  is  not  important  and  can  be  neglected  while  B 
and  F  must  be  considered,  the  line  ABEHD  has  a 
maximum  gradient  of  5  in  100,  and  connects  A  with 
the  points  BF  and  D  with  a  minimum  total  length  of 
road  (being  less  than  the  valley  line  first  considered). 

When  B  and  C  must  both  be  considered  as  of  im- 


LOCATION   OF  COUNTRY   ROADS.  6 1 

portance  as  well  as  F  and  D,  the  lines  ABCHE  and 
HD  will  give  a  ruling  gradient  of  5  in  100  to  both  F 
and  D,  and  passing  through  B  and  C  with  a  somewhat 
longer  line  than  in  the  last  case. 

This  arrangement  would  make  the  length  of  haul 
from  A  to  D  and  F,  each  longer  than  by  the  first  line 
considered  ;  but  the  gradient  to  D  would  be  lighter,  and 
the  total  length  of  road  to  be  constructed  and  main- 
tained would  be  less. 

In  case  the  points  B  and  C  are  both  unimportant, 
and  the  line  through  the  valley  may  be  neglected,  the 
line  AGFD  provides  a  ruling  gradient  of  4  in  100  from 
A  to  both  F  and  Dy  and  connects  them  with  each 
other,  with  about  the  same  length  as  the  shortest  $$ 
gradient.  When  the  point  /  must  be  taken  into  ac- 
count, this  line  may  be  connected  with  /  by  the  line 
GI  having  a  gradient  of  4  in  100.  This  would  give  the 
shortest  line  of  uniform  gradient  to  connect  A  with  the 
three  points  /  F  and  D,  and  possibly  a  desirable  line  to 
construct  when  the  line  through  the  point  7  is  impor- 
tant, even  if  the  valley  road  from  A  to  B  is  also  neces- 
sary. 

The  lines  upon  the  side  slopes  are  usually  more  ex- 
pensive to  construct  than  the  valley  lines,  and  the  dif- 
ferences of  first  cost  of  the  various  lines  must  of  course 
be  considered.  The  importance  of  a  difference  in  ex- 
pense of  construction  depends  upon  the  traffic  to  be 
hauled  over  the  road  and  the  kind  of  surface  to  be 
used.  Where  a  broken-stone  or  gravel  road  is  to  be 
constructed  at  considerable  expense,  the  difference  of 
cost  due  to  a  change  of  location  is  relatively  less  im- 
portant as  being  a  less  percentage  of  the  whole  cost, 
while  the  difference  of  tractive  effort  due  to  grade  is 


62        A  TEXT-BOOK  ON   ROADS   AND   PAVEMENTS. 

of  more  importance,  as  being  a  higher  percentage  of 
that  upon  the  level,  than  would  be  the  case  with  an 
ordinary  earth  road. 

As  is  easily  seen  from  the  above  the  choice  of  a 
location  for  a  road,  while  depending  upon  principles 
easily  stated,  is  in  reality  a  matter  requiring  the  use  of 
judgment,  and  is  not  readily  reducible  to  a  financial 
comparison  stated  in  money  values,  because  the  data 
concerning  the  volume  of  the  traffic  and  the  cost  of 
conducting  it  can  be  determined  only  very  roughly, 
and  contains  many  elements  of  error.  For  purposes  of 
comparison  to  aid  the  judgment,  approximate  data 
may  often  be  assumed  or  determined  by  a  study  of  the 
localities  affected.  In  some  cases  observations  may  be 
made  of  the  number  of  teams  of  different  classes  pass- 
ing certain  points  within  certain  times,  to  give  a  basis 
for  estimation  of  the  annual  volume  of  traffic.  In 
other  cases,  the  annual  hauling  traffic,  which  is  usually 
the  most  important  portion  of  the  traffic  in  considering 
location,  may  be  estimated  from  the  known  interests  of 
the  locality.  Thus,  if  the  produce  of  a  certain  section 
of  farming  country  must  be  hauled  over  a  given  road 
to  market,  the  amount  of  this  produce  may  be  esti- 
mated from  the  acreage,  and  the  relative  number  of 
loads  upon  different  grades  then  determined.  The 
cost  per  load  over  the  road  would  then  need  to  be  as- 
sumed in  order  to  find  the  annual  value  of  a  reduction 
of  grade. 

In  the  same  manner,  the  effect  of  changes  of  length 
and  in  the  amount  of  rise  and  fall  may  be  found  as 
indicated  in  Arts.  17  and  18. 

All  of  these  items  must  be  combined  to  find  the  rela- 
tive total  costs  of  transportation  for  each  route.  The 


LOCATION   OF  COUNTRY   ROADS.  63 

cost  of  construction  and  of  maintenance  for  each  line 
must  then  be  estimated,  and  that  line  is  the  most  ad- 
vantageous which  makes  the  sum  of  the  annual  charges 
and  the  interest  on  the  first  cost  a  minimum.  Where 
several  lines  of  traffic  are  to  be  considered  together  as 
in  Fig.  14,  the  cost  of  conducting  all  of  the  traffic  by 
each  system  of  lines  that  may  be  employed  must  be 
considered,  the  entire  cost  being  made  a  minimum  for 
the  system  to  be  adopted. 

ART.  23.    CHANGING  EXISTING  LOCATIONS. 

The  problem  that  arises  oftener  than  any  other  in 
country-road  location  is  that  of  improving  short 
stretches  of  road,  where,  owing  to  defective  location, 
the  grades  are  unnecessarily  heavy,  the  length  unneces- 
sarily great,  or  the  ground  over  which  the  road  may 
pass  such  as  to  make  its  maintenance  in  good  con- 
dition difficult  and  expensive.  The  first  of  these  is 
the  most  common  defect  of  ordinary  country  roads,  as 
shortness  of  distance  has  very  commonly  been  obtained 
by  the  disregard  of  the  desirability  of  light  gradients, 
which  in  very  many  cases  are  easily  obtainable. 

The  principles  to  be  observed  and  methods  of  pro- 
ce,dure  in  making  the  new  location  are  exactly  the 
same  as  in  an  original  location,  save  that  in  this  case  a 
road  already  exists,  and  the  question  of  economy  is  one 
of  determining  whether  the  advantages  to  be  obtained 
in  lessened  cost  and  transportation  and  maintenance  is 
sufficient  to  warrant  the  expense  of  attaining  new 
right  of  way  and  constructing  new  road. 

In  Fig.  15  is  given  an  example  that  is  frequently  met 
in  practice,  where  the  existing  road  abed  runs  oye^CfH^T 


64       A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

point  of  a  hill,  with  heavy  gradient,  while  a  line  of  very 
much  lighter  gradient  might  be  located  around  the 
base  of  the  hill  through  the  pass  at  e,  giving  a  greater 
length  of  road,  but  much  less  rise  and  fall.  The  line 
bed  in  the  figure  has  a  length  about  800  feet  greater,  a 
rise  and  fall  70  feet  less,  and  a  maximum  gradient  one 
half  as  steep  as  the  line  bed.  These  relations  are 
shown  in  the  profile  in  Fig.  15. 

If  the  road  in  question  be  a  common  earth  road, 
i  foot  of  rise  and  fall  may  be  taken  as  equivalent,  in  the 
work  required  to  haul  a  load  over  it,  to  20  feet  of  dis- 
tance, and  the  70  feet  saved  by  the  new  location  would 
be  equivalent  to  1400  feet  of  distance.  Hence,  the 
line  bed  may  be  considered  as  having  an  equivalent 
length  for  purposes  of  traffic  1400  —  800  =  600  feet 
shorter  than  the  line  bed.  In  addition  to  this,  loads 
may  be  taken  over  the  new  line  in  direction  b  to  d 
more  than  double,  and  in  direction  from  d  to  b  triple, 
in  weight  those  that  can  be  taken  by  the  same  power 
over  the  old  line. 

A  further  improvement  of  the  line  may  also  be 
possible,  if  the  new  line  can  join  the  old  one  at  a  point 
lower  down  than  $,  by  running  a  lighter  gradient  than  5 
in  100  from  the  point  e.  Thus  the  line  efa  would  give 
an  uniform  gradient  of  4$,  but  would  require  the  con- 
struction of  more  new  line. 

In  considering  changes  of  location,  it  is  also  neces- 
sary to  take  into  account  the  interests  of  adjoining 
owners.  Houses  and  buildings  are  largely  located  with 
reference  to  the  existing  position  of  the  roads,  and 
changes  in  the  position  of  a  road  may  involve  injury  to 
such  property.  The  question  then  becomes  largely 
one  of  sacrificing  the  interests  of  the  users  of  the  road, 


66        A   TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

or  those  of  the  adjoining  owners — a  question  that 
should  be,  but  commonly  is  not,  decided  by  consider- 
ing what  will  be  of  most  advantage  to  the  general  com- 
munity. 


CHAPTER    IV. 
IMPROVEMENT   OF   COUNTRY   ROADS. 

ART.  24.    NATURE  OF  IMPROVEMENTS. 

ORDINARY  country  roads  may  be  classified  as  earth 
roads,  gravel  roads,  and  broken-stone  roads.  The 
larger  number  of  common  roads  throughout  this  coun- 
try belong  of  necessity  to  the  first  class.  In  a  few  of 
the  more  enterprising  communities  the  more  important 
roads  are  constructed  of  gravel  or  broken  stone. 

The  percentage  of  roads  of  the  better  class  is,  how- 
ever, very  small  and  although  there  has  recently  been 
a  distinct  improvement  in  this  particular,  the  inability 
of  rural  communities  to  at  once  raise  the  funds  neces- 
sary for  the  general  construction  of  first-class  new 
roads  will  cause  their  increase  to  be  very  gradual. 

Improvement  in  country  roads  may  be  of  several 
kinds 

(1)  Changes  in  location,  by  which  better  alignment 
or  better  gradients  may  be  obtained,  or  by  which  the 
natural  conditions  of  surface  or  drainage  may  be  im- 
proved.    This  has  been  discussed  in  Chapter  III. 

(2)  Reconstruction  of  the  road-bed,  as  in  regrading 
steep  slopes  to  give  lighter  gradients,  or  in  raising  the 
road-bed  across   low   and   wet    places  to    provide   for 
drainage. 

67 


68        A   TEXT-BOOK   ON   ROADS   AND    PAVEMENTS. 

(3)  The  construction  of  artificial  drainage  where  a 
road   is   inclined  to    be  wet,  as   already   discussed    in 
Chapter  II. 

(4)  Improvement  of  the  surface,  which  may  consist 
in   reforming  the   surface   of  natural  earth,  or  in  the 
construction  of  an  artificial  surface  or  pavement,  the 
latter  of  which  will  be  discussed  in  separate  chapters. 

The  problem  in  common-road  improvement  is  for 
the  most  part  that  of  making  the  most  of  the  roads 
that  exist,  rather  than  reconstructing  them  with  new 
material.  The  materials  and  funds  immediately  avail- 
able must  be  used  to  secure  as  much  improvement  as 
possible. 

Earth  roads,  under  the  most  favorable  circumstances, 
do  not  usually  attain  any  high  degree  of  efficiency, 
and  are  not  economical  under  any  considerable  traffic. 
They  are,  however,  capable  of  much  improvement,  and 
if  properly  managed  need  not  become,  as  they  fre- 
quently do,  practically  useless  during  a  large  portion 
of  the  year,  although  they  are  always  more  difficult 
and  expensive  to  maintain  in  a  good  condition  than 
roads  of  a  better  and  more  permanent  construction. 

ART.  25.    EARTHWORK. 

Improvements  to  the  road-bed  of  an  existing  coun- 
try road  may  have  for  their  object  the  reduction  of 
gradient  upon  steep  inclinations,  by  cutting  the  ma- 
terial from  the  road-bed  and  lowering  the  surface  of 
the  road  on  the  upper  part  of  the  grade,  and  filling  in 
correspondingly  on  the  lower  part,  or  they  may  be 
intended  to  provide  better  drainage  by  raising  the 
road  across  low  ground. 


IMPROVEMENT  OF   COUNTRY    ROADS.  69 

In  the  construction  of  new  roads,  the  formation  of 
the  road-bed  consists  in  bringing  the  surface  of  the 
ground  to  the  grade  adopted  for  the  road.  This  grade 
should  be  carefully  established  upon  an  accurate  pro- 
file of  the  line,  in  such  manner  as  to  give  as  little 
earthwork  as  possible,  both  to  render  the  cost  of  con- 
struction low,  and  to  avoid  unnecessarily  marring  the 
appearance  of  the  country  in  vicinity  of  the  road. 
The  most  desirable  position  of  the  grade  line  is 
usually  that  which  make  the  amounts  of  cut  and  fill 
about  equal  to  each  other,  especially  where  room  for 
borrow-pits,  or  spoil-banks,  would  be  expensive,  anc 
it  is  desirable  to  make  the  embankment  for  the  most 
part  of  the  material  taken  from  the  road  excavations. 
On  side-hill  work,  one  side  of  the  road  is  commonly 
in  cut  and  the  other  in  fill,  and  where  the  side  slopes 
are  steep,  it  is  usually  better  to  make  the  road  mostly 
in  cut  on  account  of  the  difficulty  of  forming  stable 
embankments  on  steep  ground. 

Where  embankments  are  to  be  constructed,  the  sur- 
face of  the  ground  should  be  cleared  of  all  vegetable 
matter  and  soft  material  before  beginning  the  placing 
of  the  earth-filling,  in  order  to  give  a  firm  base  to  the 
bank  and  permit  it  to  bond  with  the  earth  below. 
The  material  of  an  embankment  should  be  as  homo- 
geneous as  possible,  and  all  perishable  matter  should 
be  carefully  excluded  from  it.  It  should  be  deposited 
by  beginning  at  the  outside  and  working  toward  the 
middle  in  such  a  way  as  to  give  a  concave  section  to 
the  top  of  the  bank  during  construction,  which  tends 
to  prevent  sloughing  off  along  the  lines  of  the  joints 
between  the  various  layers.  It  is  also  best  to  build  an 
embankment  a  little  narrower  at  bottom 


s^y 


70       A   TEXT-BOOK   ON    ROADS   AND    PAVEMENTS- 

at  top  than  it  is  intended  to  remain,  and  afterward 
trim  down  the  edges  to  the  proper  slope. 

Earth  in  an  embankment  will  compact  closer  than 
it  is  found  in  the  natural  state.  On  an  average  it  will 
shrink  about  one  tenth  of  its  bulk.  The  allowance  to 
be  made  for  settling  in  forming  an  embankment  de- 
pends upon  the  method  of  construction.  Where  scrap- 
ers are  used,  the  earth  will  usually  be  well  compacted 
in  placing,  and  no  allowance  is  necessary ;  with  dump 
carts  or  wagons  the  compacting  is  not  so  thorough, 
and  a  small  allowance  should  be  made ;  while  when 
wheelbarrows  are  used  or  the  earth  is  thrown  into 
place  with  shovels,  an  allowance  of  10  or  12  per  cent 
must  be  added  to  the  height  of  the  embankment,  in 
order  to  allow  for  the  final  shrinkage.  Rock  occupies 
more  space  in  embankment  than  in  excavation,  and 
does  not  need  allowance  for  shrinkage. 

In  constructing  embankments  across  wet  and  un- 
stable ground,  it  is  frequently  necessary  to  form  an  arti- 
ficial foundation  upon  which  to  place  the  earth  em- 
bankment. This  may  be  accomplished  in  some  cases 
by  excavating  a  little  of  the  soft  material  and  substi- 
tuting sand  or  gravel,  or  in  other  cases  it  may  be 
advisable  to  employ  layers  of  brushwood  or  fascines 
as  a  support  for  the  embankment.  Sometimes  it  may 
be  possible  to  drain  the  soft  material  by  deep  ditches, 
so  as  to  render  it  capable  of  sustaining  the  road,  and 
in  all  cases  drainage  should  be  provided  in  so  far  as 
possible  to  make  the  embankment  more  secure. 

When  embankments  are  to  be  formed  on  sloping 
ground,  the  surface  of  the  ground  should  be  stepped 
off,  in  order  to  hold  the  earth-filling  from  sliding  upon 
the  natural  surface  at  the  line  of  contact  between  the 


IMPROVEMENT  OF  COUNTRY  ROADS.  Jl 

two,  until  it  becomes  sufficiently  settled  for  the  de- 
velopment of  cohesion  to  cause  it  to  become  one  solid 
mass. 

In  many  cases  where  roads  are  to  be  constructed 
along  steep  slopes,  it  is  found  cheaper  to  use  retaining 
walls  to  sustain  the  road  upon  the  lower  side  and  the 
earth-cutting  on  the  upper  side  than  to  cut  long  slopes 
or  form  high  embankments. 

Catch-water  drains  are  necessary  on  the  natural  sur- 
face above  the  top  of  all  high  slopes  in  cuttings  to 
prevent  the  surface-water  from  washing  down  and 
destroying  the  face  of  the  slope. 

Where  springs  are  tapped  by  a  cutting,  drains  must 
be  provided  to  remove  the  water  without  injury  to 
the  slope ;  and  where  the  subsoil  may  become  wet  in 
rainy  weather,  it  may  be  necessary  to  provide  sub- 
surface drains  along  the  slope  to  prevent  the  earth 
becoming  saturated  and  sliding  down  into  the  road- 
way. 

Slopes,  both  of  excavation  and  embankment,  are 
greatly  improved  by  being  sodded  or  sown  with  grass. 
This  aids  in  the  maintenance  of  the  slopes,  by  render- 
ing them  more  capable  of  resisting  the  abrading  action 
of  such  water  as  falls  upon  them.  It  also  greatly  im- 
prove^ their  appearance. 

The  most  important  principle  involved  in  the  forma- 
tion of  a  road-bed,  which  should  be  always  in  mind,  is 
that  earth  in  order  either  to  sustain  a  load  or  to  main- 
tain a  slope,  must  be  kept  dry,  or  at  least  prevented 
from  becoming  saturated  with  water,  as  both  the 
cohesive  and  frictional  resistances  of  earth  are  dimin- 
ished or  destroyed  when  it  becomes  wet,  and  it  is 
also  then  liable  to  the  disturbing  action  of  frost. 


72        A  TEXT-BOOK   ON   ROADS  AND    PAVEMENTS. 

ART.  26.    DRAINAGE. 

Drainage  is  especially  important  upon  earth  roads, 
because  the  material  of  the  road  surface  is  more  sus- 
ceptible to  the  action  of  water,  and  more  easily 
destroyed  by  it  than  are  the  materials  used  in  the 
construction  of  the  better  class  of  roads.  When 
water  is  allowed  to  stand  upon  the  road,  the  earth  is 
softened  and  readily  penetrated  by  the  wheels.  The 
action  of  frost  is  also  apt  to  be  more  disastrous  upon 
the  more  permeable  surface  of  the  earth  road,  having 
an  effect  to  swell  and  heave  the  roadway  and  throw 
its  surface  out  of  shape.  It  may  in  fact  be  said  that 
the  whole  problem  of  the  improvement  and  mainte- 
nance of  ordinary  country  roads  is  one  of  drainage. 

In  underdraining  an  earth  road  on  account  of  the 
permeability  of  the  surface,  provision  must  be  made 
for  carrying  off  the  water  which  penetrates  through 
the  surface,  as  well  as  that  due  to  natural  wetness  of 
the  subsoil.  The  surface  should  of  course  be  made  of 
such  form  and  material  as  to  cause  the  water  to  flow 
off  without  penetrating  as  far  as  possible ;  but  in 
damp  weather  wheels  will  mark  the  surface  somewhat, 
and  water  held  in  the  ruts  so  formed  will  soak  into  the 
earth,  and  unless  at  once  removed  below  soften  it  so 
that  the  next  wheel  makes  a  deeper  rut,  with  the 
final  result  of  destroying  the  form  of  the  road  as  well 
as  its  power  to  sustain  the  loads  that  come  upon  it. 

The  necessity  for  the  application  of  artificial  sub- 
drainage  in  any  case  is  determined  by  local  conditions, 
the  character  of  the  soil,  and  natural  drainage.  An 
examination  of  the  line  of  the  road  in  wet  weather, 
observing  whether  water  stands  upon  the  ground,  the 


IMPROVEMENT   OF  COUNTRY   ROADS.  73 

direction  of  flow  of  surface-water,  and  whether  that 
which  penetrates  the  ground  drains  away  quickly  is 
usually  an  efficient  aid  in  forming  an  opinion  as  to  the 
necessity  for  drainage. 

The  methods  employed  in  draining  are  considered 
in  Chapter  II.  Dependence  is  most  commonly  placed 
upon  shallow  side-ditches,  which  are  seldom  of  much 
value  except  to  carry  off  surface-water ;  and  even  when 
the  side-ditches  are  deep,  they  can  only  be  efficient 
for  subdrainage  when  the  soil  is  of  a  very  open, 
porous  nature.  In  other  cases  they  will  not  draw  the 
water  from  the  subsoil  under  the  middle  of  the  road, 
and  cross-drains  or  a  centre  drain  should  be  provided. 

The  common  neglect  of  proper  drainage  is  undoubt- 
edly very  largely  responsible  for  the  general  bad  con- 
dition of  country  roads. 

ART.  27.    EARTH-ROAD  SURFACE. 

The  method  which  should  be  adopted  for  the  im- 
provement of  the  surface  of  an  earth  road  depends 
upon  the  nature  of  the  material  of  which  it  may  be  com- 
posed. When  the  material  is  loose  sand,  the  surface 
will  be  more  firm  if  the  sand  be  damp  and  more 
unstable  in  dry  weather.  In  such  case  a  small  admixt- 
ure of  clay  in  the  surface  layer  may  give  cohesion  to 
the  surface  when  dry,  or  a  layer  of  clay  six  or  eight 
inches  deep  may  form  a  hard  and  comparatively 
durable  surface,  as  it  is  easily  drained  when  upon  the 
sand  road-bed. 

Clay  soils  as  a  rule  absorb  quite  freely  the  water  with 
which  they  may  be  held  in  contact,  and  soften  when 
saturated,  but  are  not  readily  permeable,  and  hence  are 


74       A  TEXT-BOOK   ON   ROADS    AND    PAVEMENTS. 

not  easily  drained  from  below.  Used  alone  they  are 
consequently  the  least  desirable  of  road  materials. 
When  dry,  clay  may  make  a  very  hard  and  durable  sur- 
face, and  it  may  give  good  results  as  a  covering  for  a  road- 
bed of  more  pervious  material,  or  it  may  form  a  stable 
road-bed  when  protected  by  a  surface  which  does  not 
soften  so  readily  and  prevents  the  surface-water  from 
reaching  the  clay  beneath.  In  building  over  clay,  sand 
or  gravel  may  frequently  be  mixed  with  the  clay  to 
form  a  surface  layer  which  will  be  less  acted  upon 
by  water.  When  rather  coarse  sand  or  small  gravel  is 
used  for  this  purpose  and  a  small  proportion  of  clay 
just  sufficient  to  bind  the  particles  of  sand  together, 
a  very  hard  and  compact  mass  is  formed,  nearly  im- 
pervious to  water  and  but  little  acted  upon  by  it.  Ma- 
terial of  this  nature  found  in  a  natural  state  is  known 
as  hard-pan,  and  is  very  stable  and  durable.  A  layer 
of  sand  a  few  inches  deep  may  also  sometimes  be 
employed  to  form  a  surface  over  a  clay  road-bed  which 
will  not  soften  in  wet  weather,  and  will  afford  protec- 
tion to  the  clay  beneath. 

When  other  material  cannot  be  obtained,  clay  roads 
are  sometimes  improved  by  burning  the  clay  so  as  to 
form  a  more  porous  material  for  use  as  a  surface  layer. 
This  method,  however,  is  somewhat  expensive,  and 
other  materials  may  usually  be  employed  at  less  cost. 

Soils  composed  of  mixtures  of  sand  and  clay  or  of 
gravel  and  clay  are  usually  easier  to  deal  with  than  clay 
itself,  and  commonly  form  the  best  natural  roads.  They 
vary  in  character  from  the  light  sandy  loams  to  heavy 
soils  partaking  very  much  of  the  nature  of  clay.  The 
sandy  soils  take  up  water  readily  and  become  soft  when 
wet ;  but  they  are  pervious  and  easily  drained,  and  they 


IMPROVEMENT  OF  COUNTRY   ROADS.  75 

may  be  compacted  into  a  firm  surface  in  dry  weather. 
The  heavier  soils  take  up  water  readily  and  become  soft 
when  wet,  but  are  less  pervious  and  drained  with  more 
difficulty,  though  much  more  easily  than  a  clay. 

The  material  of  a  road  surface  should  always  be  such 
as  may  be  compacted  to  a  firm  and  hard  surface. 
It  should  not,  therefore,  be  formed  of  the  soft  ma- 
terial which  may  be  washed  into  the  gutters.  The  sur- 
face must  be  formed  with  a  crown  at  the  middle  suffi- 
cient to  shed  the  water  which  may  fall  upon  it  into  the 
gutters,  and  prevent  water  from  standing  upon  the 
road.  The  slope  necessary  to  shed  the  water  readily 
is  about  i  in  20,  and  the  most  desirable  section  is  usu- 
ally that  composed  of  two  planes  of  equal  inclination, 
rounded  off  in  the  middle  and  sloping  uniformly  to  the 
sides,  as  shown  in  Fig.  16. 

In  the  construction  of  an  earth-road  surface,  road- 


FIG.  16. 

machines  or  road-scrapers  may  often  be  employed  to 
advantage,  especially  when  no  grading  is  to  be  done 
other  than  giving  the  road  the  proper  crown.  The  gut- 
ters may  thus  be  formed,  and  the  surface  shaped  up 
with  comparatively  little  labor. 

After  the  material  is  in  position,  the  surface  should  be 
compacted  to  the  required  form  by  rolling  with  as 
heavy  a  roller  as  may  be  available.  This  is  a  very  im- 
portant matter  in  attempting  to  form  a  satisfactory 
earth  road,  and  is  almost  indispensable  to  success.  If 
the  loose  earth  be  thrown  into  the  middle  of  the  road 
to  be  compacted  by  the  wheels  of  traffic,  the  action  of 


76       A   TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

the  wheels  will  be  to  cut  it,  or  at  least  to  pack  it  in  a 
very  uneven  manner,  producing  a  surface  uneven  and 
full  of  ruts,  which  will  hold  water  and  ultimately  cause 
the  destruction  of  the  road.  In  case,  however,  the  sur- 
face be  properly  rolled,  it  may  usually  be  made  suffi- 
ciently firm  to  hold  up  the  wheels  and  retain  its  form 
under  the  traffic,  and  if  kept  free  from  ruts  until  thor- 
oughly compacted  will  thus  be  rendered  much  more 
capable  of  resisting  the  penetration  of  water  and  shed- 
ding  it  into  the  side  gutters. 

ART.  28.    GRAVEL  ROADS. 

Gravel  roads  may  vary  from  that  in  which  a  thin 
coating  of  gravel  is  used  as  a  wearing-surface  upon 
an  earth  road  to  that  in  which  gravel  is  used  as  a  sur- 
face for  the  heavy  Telford  construction  of  a  road  of  the 
first  class.  These  latter  constructions  will  be  treated  in 
Chapter  V,  under  the  head  of  "  Broken-stone  Roads." 

In  the  improvement  of  a  country  road,  where  the 
construction  of  a  good  Telford  or  Macadam  road  can- 
not be  undertaken,  a  surface  of  gravel  may  frequently 
be  used  to  advantage,  giving  much  better  results  than 
could  be  obtained  with  the  surface  of  earth.  Even 
a  light  layer  of  gravel  may  frequently  prove  of  very 
great  benefit. 

Where  the  subsoil  is  of  a  porous  nature  and  well 
drained,  a  layer  of  three  or  four  inches  of  gravel,  or  some- 
times even  less,  well  compacted,  will  constitute  a  very 
considerable  improvement ;  especially  if,  as  is  usual  with 
these  light  soils,  the  nature  of  the  material  of  the  road- 
bed is  particularly  unsuitable  for  the  wearing-surface, 
difficult  to  compact  sufficiently  to  shed  water,  and  likely 
to  become  soft  when  wet. 


IMPROVEMENT   OF   COUNTRY   ROADS.  77 

Where  the  road-bed  is  of  clay  a  deeper  layer  of 
gravel,  at  least  6  inches,  is  usually  required  for  effective 
work,  as  the  gravel  must  be  deep  enough  to  prevent 
the  weight  of  the  traffic  forcing  the  surface  layer  into 
weak  places  in  the  clay  beneath,  and  also  to  effectually 
prevent  the  surface-water  from  reaching  the  clay. 

Gravel  to  be  used  on  roads  should  be  sharp  and 
comparatively  clean.  In  order  to  bind  well  in  the  road 
it  should  usually  have  a  small  admixture  of  clay. 
Gravel  in  which  the  stones  are  round  or  oval,  such 
as  is  commonly  found  in  the  beds  of  streams,  is  unfit 
for  the  construction  of  roads;  the  small  stones  of  which 
it  is  composed,  having  no  angular  projections,  will  not 
bind  together,  and  even  when  mixed  with  clay  may  turn 
freely,  and  will  be  difficult  to  firmly  bed  in  position. 
Pit  gravel  is  usually  more  sharp,  but  is  frequently  found 
mixed  with  considerable  earth,  which,  as  well  as  the 
larger  stones  should  be  removed  by  screening  before 
using  the  gravel.  Screens  of  ij  inch  and  J  inch  open- 
ings may  be  employed  for  this  purpose — that  material 
only  which  passes  the  larger  and  is  rejected  by  the 
smaller  being  used  in  the  work. 

In  the  construction  of  a  road  with  gravel  surface  the 
road-bed  should  first  be  brought  to  the  proper  grade, 
with  a  form  of  cross-section  the  same  as  that  to  be 
given  the  finished  road.  The  gravel  is  then  placed  upon 
it  and  rolled  to  a  surface,  or  left  to  be  compacted  by 
the  traffic.  It  is  always  advantageous  when  possible  to 
compact  the  road  by  rolling.  The  road-bed  should  be 
well  rolled  before  placing  the  gravel,  and  the  gravel 
surface  afterward.  A  smooth  hard  surface  may  thus  be 
produced,  upon  which  the  wheels  of  loaded  vehicles 
may  roll  without  producing  any  visible  impression. 


78        A   TEXT-BOOK  ON   ROADS  AND  PAVEMENTS. 

Where  the  compacting  of  the  road  is  left  to  the  traffic 
constant  watchfulness  is  necessary  to  prevent  unequal 
wear  and  the  formation  of  ruts. 


ART.  29.    MAINTENANCE  OF  COUNTRY  ROADS. 

The  maintenance  of  a  country  road  in  good  con- 
dition is  a  matter  requiring  constant  care  and  watch- 
fulness. Any  small  breaks  in  the  surface  must  be 
immediately  repaired,  and  ruts  filled  and  smoothed 
before  they  become  serious. 

The  work  required  to  keep  a  road  in  repair  depends 
upon  the  nature  of  the  surface  and  the  efficiency  of 
the  drainage.  A  well-constructed  road  of  good  ma- 
terial will  be  much  easier  and  less  expensive  to  keep  in 
repair  than  one  in  which  the  surface  is  not  firm  enough 
to  resist  the  cutting  action  of  the  traffic,  or  which  has 
a  surface  compound  of  material  readily  softened  by  the 
action  of  water  which  may  fall  upon  it. 

Earth  roads  under  the  most  favorable  conditions  are 
expensive  to  maintain,  and  especially  so  under  the 
common  system  of  repairing  once  or  twice  a  year,  or 
at  long  intervals.  This  system  is  not  only  costly  in  the 
work  required,  which  usually  amounts  to  a  practical  re- 
construction of  the  road  each  time  repairs  are  under- 
taken ;  but  it  is  ineffectual  in  that  the  road  for  the  larger 
portion  of  the  time  is  out  of  repair  and  in  bad  condi- 
tion, even  if  the  work  of  construction  has  been  well 
done,  which  is  not  usually  the  case  where  this  method 
obtains. 

The  only  way  to  keep  an  earth  road  in  good  con- 
dition is  by  the  employment  of  men  whose  business  it 
shall  be  to  continually  watch  the  road,  and  make  such 


IMPROVEMENT  OF  COUNTRY   ROADS.  79 

small  repairs  as  may  be  necessary  from  time  to  time. 
The  small  washes  that  may  occur  during  heavy  storms, 
ruts  formed  by  wagons  travelling  in  the  same  track,  or 
in  passing  over  soft  spots  when  the  road  is  wet,  or 
any  small  breaks  in  the  surface  of  the  road,  should  be 
at  once  attended  to  and  carefully  filled  with  new 
material. 

Where  small  repairs  are  needed  over  a  considerable 
area  of  the  road  the  use  of  the  road-machine  is  usually 
advantageous,  as  giving  an  easy  method  of  smoothing 
up  the  surface.  The  use  of  a  roller  is  also  nearly 
always  of  value,  both  to  assist  in  smoothing  the  surface 
to  the  proper  form,  and  to  give  compactness  to  it.  By 
the  occasional  use  of  these  machines  through  the  dry 
seasons  a  road  may  be  kept  crowning  and  hard,  so  that 
most  of  the  rainfall  will  be  quickly  shed  off  into  the 
side  gutters  without  injury  to  the  road. 

When  there  are  long-continued  rains,  or  when  the  ice 
and  snow  of  winter  are  melting  in  the  spring,  an  earth- 
road  surface  will  necessarily  be  more  or  less  softened 
and  cut  by  passing  vehicles ;  and  at  such  times  a  road 
of  this  character  cannot  be  maintained  in  the  same 
condition  as  in  dry  weather,  or  in  the  condition  which 
would  be  possible  with  a  less  permeable  surface,  but  if 
at  the  beginning  of  the  wet  period  it  be  in  proper  form 
and  if  the  subdrainage  be  efficient,  the  injury  to  the 
road  as  well  as  the  duration  of  the  bad  condition,  will 
be  reduced  to  a  minimum.  As  soon  as  possible  after 
such  a  wet  time,  the  roads  should  be  gone  over  with 
the  scraper  and  put  into  proper  form,  and  then  rolled 
down  hard.  It  is  advantageous  to  have  this  done  be- 
fore the  ground  becomes  thoroughly  dry  and  hard,  as 


80        A  TEXT-BOOK   ON   ROADS   AND    PAVEMENTS. 

it  will  work  more  freely,  and  may  be  compacted  much 
closer  by  the  roller  than  afterward. 

In  repairing  a  road  where  the  gutter  is  filled  with 
soft  material  which  must  be  removed  to  afford  a  free 
channel  for  the  surface-water,  this  soft  material  should 
not  be  scraped  upon  the  middle  of  the  road,  as  it  will 
not  form  a  good  wearing-surface.  Where,  however, 
a  road  is  in  fairly  good  condition,  and  merely  needs 
a  little  smoothing  up,  it  is  desirable  to  work  from  the 
gutter,  scraping  the  material  lightly  toward  the  middle 
until  the  proper  crown  is  obtained. 

The  difficulty  and  cost  of  maintaining  a  road  will  of 
course  vary  with  the  nature  of  the  traffic  that  passes 
over  it.  A  road  for  light  driving  will  be  much  easier 
to  keep  in  repair  than  one  used  by  heavy  loads,  and 
as  the  amount  of  heavy  traffic  becomes  greater  the 
economy  of  the  earth-road  surface  is  lessened,  and  the 
desirability  of  the  substitution  of  a  more  durable  wear- 
ing-surface increases. 

The  width  of  the  wheel-tires  upon  which  the  loads 
are  carried  is  also  important  in  its  effect  upon  the  cost 
of  keeping  a  road  in  repair.  Narrow  tires  cut  and  rut 
the  surface  of  a  road,  while  those  of  sufficient  width  act 
as  rollers  to  compact  the  material.  For  the  best  results 
the  tires  should  be  as  wide  as  possible,  and  the  front 
and  rear  wheels  of  a  wagon  should  not  run  in  the  same 
track.  The  lighter  tractive  effort  required  for  wide 
tires  on  compressible  road  surfaces  has  been  referred 
to  in  Art.  2. 

ART.  30.  WIDTH  OF  COUNTRY  ROADS. 

The  width  of  the  roadway  upon  country  roads 
should  be  only  sufficient  to  provide  space  for  the  easy 


IMPROVEMENT  OF  COUNTRY  ROADS.      8 1 

conduct  of  the  traffic.  For  roads  of  ordinary  traffic 
this  requires  only  that  there  shall  be  room  for  teams 
moving  in  opposite  directions  to  freely  pass  each 
other.  An  available  width  of  16  feet  is  ample  for  this 
purpose,  and  14  feet  is  often  sufficient.  Too  great 
width  in  the  roadway  causes  an  unnecessary  increase 
in  the  cost  of  constructing  and  maintaining  the  road. 
Where  the  road-surface  is  of  earth  it  will  be  much 
easier  to  drain  it  if  it  be  narrow  than  if  it  be  wide.  If 
deep  side-ditches  be  depended  on  for  subdrainage,  the 
nearer  they  are  together  the  more  effectively  will  they 
drain  the  subsoil  under  the  middle  of  the  road.  Side 
ditches  must,  however,  be  far  enough  apart  so  that  a 
berm  may  be  left  on  each  side  between  the  travelled 
part  of  the  road  and  the  ditches.  Thus  in  Fig.  7,  p.  31,  if 
the  macadamized  portion  represents  the  travelled  part 
of  the  road,  the  berm  between  that  portion  and  the 
ditches  could  be  sown  with  grass  and  show  the  line  of 
the  road  as  a  guide  to  travel. 

When  covered  drains  are  used  for  subdrainage  the 
gutters  at  the  side  may  be  made  shallow  and  placed 
next  the  travelled  part  of  the  road,  giving  much  less 
surface  to  maintain  and  greater  efficiency  to  the  drain- 
age than  in  a  wider  road.  Such  sections  are  shown 
for  earth  roads  in  Figs.  6  and  16.  Fig.  4  shows  a 
similar  construction  with  side-drains  under  the  gutter 
and  a  broken-stone  surface.  Fig.  17  shows  the 
ordinary  form  of  a  country  road  with  broken-stone 
surface.  On  important  roads  the  paved  portion  is 
commonly  16  or  18  feet  in  width,  but  on  roads  of 
lesser  importance  it  may  be  less,  and  under  light  traffic 
a  width  of  10  feet  maybe  sufficient — teams,  when i 
sary,  turning  out  upon  the  sod  to  pass.  Where^less 


LH 


82        A   TEXT-BOOK    ON    ROADS   AND    PAVEMENTS. 

pervious  covering  is  employed,  as  with  gravel  or  broken 
stone,  width  will  not  have  the  same  tendency  to  render 
drainage  ineffectual  as  in  the  case  of  an  earth  road, 
because  comparatively  little  water  will  pass  through 
the  road-surface  to  the  subsoil.  The  cost  of  main- 

^ 


FIG.  17. 

tenance  may  not,  therefore,  be  so  materially  affected 
by  the  width,  although  the  cost  of  construction,  and 
hence  the  length  of  road,  that  may  be  built  for  a  give 
sum  will  be  directly  dependent  upon  it. 

While  the  improved  portion  of  the  road  should  be 
as  small  as  is  consistent  with  the  proper  discharge  of 
the  duty  required  of  it,  the  available  right  of  way  need 
not  be  so  restricted,  but  should  be  laid  out  wide 
enough  to  permit  of  the  widening  of  the  used  portion 
when  necessary,  and  allow  room  at  the  sides  for  pedes- 
trians, with  a  grass  border  and  line  of  trees.  When 
trees  are  planted  along  the  roadway  they  should  not 
be  placed  so  as  to  form  a  dense  shade  over  any  portion 
of  the  travelled  road,  although  a  moderate  shade  is 
not  a  disadvantage,  and  care  should  be  used  that  they 
are  not  near  enough  to  a  covered  drain  to  permit  the 
roots  to  grow  into  the  drain  and  choke  it. 

ART.  31.  ECONOMIC  VALUE  OF  ROAD  IMPROVEMENT. 

The  value  of  a  road  improvement  to  a  community 
and  the  amount  of  money  that  may  reasonably  and 
profitably  be  expended  in  the  construction  and  main- 
tenance of  common  roads  is  a  subject  the  discussion  of 


IMPROVEMENT  OF  COUNTRY   ROADS.  83 

which  leads  different  persons  to  widely  different  con- 
clusions, depending  upon  the  point  of  view  and  the 
data  assumed. 

The  economic  principles  involved  in  a  choice  of 
location  have  already  been  discussed  in  Chapter  III ; 
and  the  general  value  of  any  other  improvement,  in  so 
far  as  it  relates  to  the  economic  conduct  of  the  traffic, 
may  be  considered  in  the  same  manner.  Any  improve- 
ment, either  in  position  or  surface,  that  has  the  effect 
of  increasing  the  loads  that  may  be  taken  over  a  road 
by  a  given  power  lessens  the  number  of  loads  neces- 
sary to  carry  the  traffic,  and  effects  a  saving  in  time 
and  labor  of  men  and  teams,  which  may  reasonably  be 
considered  to  have  the  same  money  value  as  the  time 
used  in  the  work. 

On  ordinary  country  roads  in  dry  weather,  the  amount 
of  load  that  can  be  hauled  is  usually  determined  rather 
by  the  grades  than  by  the  nature  of  the  surface.  Un- 
less the  gradients  are  very  light  the  amount  of  load 
that  can  be  carried  on  a  broken-stone  surface  does  not 
differ  greatly  from  what  may  be  taken  on  a  dry  and 
hard  earth  road.  In  improving  a  road  by  substituting 
a  hard  surface  for  a  surface  of  earth  the  gradients  and 
location  should  therefore  always  be  carefully  studied, 
with  a  view  to  deriving  the  full  practical  benefit  from 
the  hard  surface  in  the  light  traction  that  it  may  re- 
quire with  easy  ruling  gradients. 

It  is  in  wet  and  muddy  weather  that  improved  sur- 
faces have  their  chief  advantage  over  earth  roads,  and 
the  main  object  of  introducing  hard  and  impermeable 
surfaces  is  to  eliminate  the  period  when  ordinary  earth 
roads  are  apt  to  be  muddy  and  practically  useless  for 
the  purposes  of  transportation,  and  to  substitute  a 


84        A    TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

road  that  may  be  used  at  any  season.  Systematic 
drainage  has  a  similar  object.  To  a  farming  com- 
munity the  economic  advantage  of  a  road  uniformly 
good  at  all  seasons  is  greater  than  might  appear  at 
first  glance.  It  may  in  many  instances  amount  prac- 
tically to  a  saving  equal  to  nearly  the  entire  cost  of 
hauling,  by  permitting  the  work  to  be  done  at  times 
when  other  work  is  impossible,  thus  making  men  and 
teams  available  for  other  duty  in  good  weather.  The 
ability  to  use  a  road  at  any  season  is  also  of  advantage 
in  the  independence  of  weather  that  will  make  it  pos- 
sible to  take  advantage  of  the  condition  of  the  markets 
in  the  disposal  of  produce  or  purchase  of  supplies. 

The  nature  of  the  roads  has  likewise  an  important 
effect  upon  the  social  life  of  the  people  in  a  rural  dis- 
trict, and  has  much  to  do  with  the  desirability  of  a 
locality  as  a  place  of  residence.  These  items  all  have 
a  real  importance,  which,  while  difficult  to  estimate  in 
money  values,  show  at  once  in  the  fact  that  prices  of 
country  property  are  largely  affected  by  them. 

The  nature  of  the  country  roads  affect  the  towns  to 
which  the  country  is  tributary  as  well  as  the  country 
itself.  They  directly  affect  trade  in  seasons  of  bad 
weather,  both  in  regulating  the  demand  for  supplies 
for  country  consumption  and  in  controlling  the  supply 
of  produce  which  is  available  for  market ;  indirectly 
also  the  prosperity  of  a  rural  district  means  that  of  its 
trade  centre. 

All  of  these  points  must  be  considered  in  any  at- 
tempt to  arrive  at  any  proper  conception  of  the  advan- 
tages of  a  proposed  improvement.  In  any  particular 
case  the  local  interests  will  determine  the  relative  im- 
portance of  the  various  elements,  and  a  careful  analysis 


IMPROVEMENT   OF   COUNTRY   ROADS.  85 

of  the  trade  that  does  pass  over  the  road  and  that 
would  pass  over  it  under  different  conditions  will 
enable  a  judgment  to  be  formed  as  to  the  value  of 
improvements. 

The  money  spent  in  road  improvement  is  to  be  con- 
sidered as  an  investment,  which  will  return  annual 
interest  to  the  community  in  reduced  costs  of  trans- 
portation and  greater  freedom  of  traffic  and  travel. 

ART.  32.    SYSTEMS  OF  ROAD  MANAGEMENT. 

Several  different  systems  for  managing  the  work  of 
constructing  and  repairing  country  roads  have  been 
proposed  or  are  in  use  in  various  places.  These  sys- 
tems differ  in  the  placing  of  the  control  of  the  roads 
and  in  the  methods  adopted  for  providing  funds. 

The  control  of  the  roads  under  the  various  systems 
may  be  vested  in  the  national  government,  in  the  vari- 
ous State  governments,  in  county  or  parish  organiza- 
tions or  in  townships  or  districts.  In  regard  to  the 
location  of  control  and  responsibility,  it  may  be  re- 
marked that  there  are  two  points  to  be  kept  in  view. 

ist.  In  order  that  the  work  may  be  economically 
conducted,  the  section  of  country  included  under  one 
control  should  be  sufficient  to  warrant  the  permanent 
employment  of  a  man,  or  corps  of  men,  whose  business 
it  shall  be  to  continually  look  after  the  roads,  study 
their  needs,  and  systematically  conduct  their  improve- 
ment. It  should  admit  of  the  ownership  and  use  of 
labor-saving  machinery  for  the  economical  execution 
of  the  work,  but  should  not  be  large  enough  to  require 
an  elaborate  and  complicated  organization. 

2d.  The  control  of  road  work  should  be  so  arranged 
that,  as  nearly  as  possible,  all  of  the  interests  directly 


86        A   TEXT-BOOK   ON   ROADS   AND    PAVEMENTS. 

affected  by  the  condition  of  any  road  shall  have  a 
voice  in  its  management  and  contribute  to  its  support. 

Common  roads  are  essentially  local  in  their  character 
and  are  not  usually  employed  as  lines  of  continuous 
transportation  over  any  considerable  distance.  They 
are  not,  therefore,  of  State  or  national  importance  as 
lines  of  communication,  although  as  factors  in  the 
general  welfare  of  the  people  they  must,  of  course,  like 
all  other  such  factors,  be  of  general  interest  and  con- 
cern to  both  State  and  nation. 

The  nation,  and  in  most  cases  in  this  country  the 
State,  is  too  large  an  unit  to  assume  direct  control  of 
road  work.  In  general,  the  interests  over  so  large  an 
area  are  so  varied,  and  the  requirements  so  different, 
as  to  prevent  a  harmonious  and  successful  organization 
of  such  work  with  a  probability  of  economical  adminis- 
tration. In  some  cases,  however,  such  control  might 
be  wise  and  proper,  and  the  recognition  of  the  impor- 
tance of  road  improvement  to  the  general  welfare  of 
the  State,  through  the  payment  by  the  State  of  a 
portion  of  the  cost  of  permanent  improvements,  has  in 
some  instances  proved  a  powerful  stimulus  to  local 
action. 

The  control  of  road  management  by  towns  and  small 
districts  is  nearly  always  inefficient  because  the  organ- 
ization is  too  small  to  support  a  proper  management 
or  provide  the  necessary  appliances  for  economic  work. 
Under  this  system  the  man  in  charge  of  the  roads  is 
usually  engaged  in  other  work,  he  is  not  a  road  engineer, 
and  can,  and  is  expected  to,  give  but  little  attention  to 
the  road  work.  This  system  of  control  is  also  usually 
unfair,  except  in  case  of  roads  intended  for  the  accom- 
modation of  the  local  district  only.  For  instance,  a 


IMPROVEMENT  OF  COUNTRY  ROADS.      8? 

road  passing  through  a  town  may  be  a  thoroughfare 
for  the  towns  upon  each  side.  The  principal  traffic 
may  be  this  through-trade  to  points  beyond  the  limits 
of  the  town  in  which  the  road  is  situated.  The  cost  of 
keeping  up  this  road  is  largely  due  to  outside  traffic, 
and  the  intermediate  town  should  not  be  required  to 
bear  all  the  expense  of  maintenance.  On  the  other 
hand,  the  interests  of  the  towns  whose  trade  passes 
over  the  road  are  largely  affected  by  its  nature,  and 
the  people  of  these  towns  should  be  permitted  a  voice 
in  determining  the  character  of  the  road.  Most  of  the 
more  important  roads  of  every  vicinity  pass  thus 
through  several  towns,  and  the  system  of  improvement 
by  small  districts  works  injustice  both  ways — upon 
those  who  are  obliged  to  keep  a  road  for  the  use  of 
others  and  upon  those  who  are  obliged  to  use  a  road 
they  cannot  cause  to  be  kept  in  proper  condition. 

County  management  seems  more  successful  in  this 
country  than  any  other,  as  a  county,  or  two  counties 
combined  if  necessary,  is  usually  strong  enough  to 
secure  intelligent  management  and  homogeneous 
enough  to  have  common  interests. 

The  proper  management  of  the  common  roads  in 
any  community  requires  both  experience  and  intelli- 
gence. A  man  to  be  efficient  in  such  work  must  be 
able  to  make  or  modify  location  where  necessary, 
judge  of  the  value  of  various  materials  for  purposes  of 
construction,  determine  the  necessity  for  and  means  to 
be  adopted  for  drainage,  and  possess  the  executive 
ability  to  manage  men  and  control  scattered  work. 
The  work  In  each  locality  is  a  problem  by  itself,  to  be 
solved  by  careful  study  of  the  requirements  of  the 
community,  taking  into  account  the  local  natural  con- 
ditions and  available  materials  and  means. 


CHAPTER   V. 
BROKEN-STONE  ROADS. 

ART.  33.    DEFINITION. 

BROKEN-STONE  roads  consist  essentially  of  a  mass  of 
angular  fragments  of  rock  deposited,  usually  in  layers, 
upon  the  road-bed  or  a  foundation  prepared  for  it,  and 
then  consolidated  to  a  smooth  and  uniform  surface  by 
means  of  a  roller  or  by  the  action  of  the  traffic  which 
passes  over  it. 

There  are  two  commonly  recognized  systems  of  con- 
structing broken-stone  roads,  differing  in  the  nature  of 
the  foundation  employed,  and  known  respectively  by 
the  names  of  the  men  who  first  introduced  them  into 
English  practice  as  Telford  roads  and  Macadam  roads. 

Each  of  these  systems  has  been  greatly  modified  in 
use  since  the  time  of  its  founder,  and  each  name  is  now 
used  to  cover  a  general  class  of  constructions  differing 
very  materially  within  itself  as  applied  in  the  practice 
of  different  engineers.  Each  of  the  systems  also  has 
its  earnest  advocates,  who  contend  for  its  exclusive  use, 
and  numerous  controversies  have  been  the  result,  at 
the  conclusion  of  which  each  party  is  "  of  the  same 
opinion  still."  The  view  taken  by  different  road- 
builders  in  this  matter,  it  may  be  remarked,  appears  to 
be  the  result  usually  of  the  local  necessities  of  the 
vicinities  in  which  they  work,  and  of  the  skill  with  which 

88 


BROKEN-STONE   ROADS.  89 

the  different  systems  have  been  applied  in  work  which 
has  come  under  their  observations.  In  road-building, 
as  in  any  other  class  of  engineering  works,  no  rigid 
rules  can  be  laid  down  for  universal  application  ;  each 
road  must  be  designed  for  the  place  it  is  to  occupy  and 
the  work  it  is  to  do. 

In  some  parts  of  this  country  natural  gravel  is  sub- 
stituted for  broken  stone  in  the  construction  of  these 
roads,  the  methods  of  construction  being  the  same  as 
in  using  broken  stone. 

ART.  34.    MACADAM  ROADS. 

Macadam  roads  as  commonly  constructed  consist  of 
two  or  more  layers  of  broken  stone,  each  layer  being 
rolled  to  a  firm  bearing  before  placing  the  next.  The 
broken  stone  is  usually  placed  directly  upon  the  earth 
road-bed. 

In  constructing  a  macadamized  roadway,  the  road- 
bed is  first  brought  to  the  proper  grade  in  the  usual 
manner,  and  rolled  to  a  uniform  surface.  The  surface 
of  the  road-bed  is  either  flat  or  raised  at  the  middle  to 
the  same  section  as  is  to  be  given  the  finished  road- 
surface.  The  inclined  form  is  usually  employed,  and 
seems  preferable  on  account  of  affording  better  drain- 
age in  case  any  water  finds  its  way  through  the  surface 
layer. 

On  village  streets  where  curb  and  sidewalks  are  em- 
ployed, this  section  of  the  road-bed  may  extend  to  the 
curbing  (as  shown  in  Fig.  5^,  but  on  country  roads  a 
bench  of  earth  should  be  left  at  the  side  between  the 
broken  stone  and  the  gutter  in  order  to  confine  the 
broken  stone  while  it  is  being  compacted,  and  of-event  ^ 

S%S''     - 

>W 


3 


90        A   TEXT-BOOK   ON    ROADS   AND    PAVEMENTS. 

the  spread  of  the  surface  materials.  The  form  of  the 
road-bed  before  placing  the  stone  would  then  be  as 
shown  in  Fig.  18,  where  the  completed  road  is  to  be  of 
the  form  given  in  Figs.  4  and  7.  Where  the  road-bed 
is  in  embankment,  it  is  common  to  construct  the  earth 
embankment  to  the  height  of  the  finished  surface,  and 
afterwards  excavate  the  material  necessary  to  admit  of 


FIG.   18. 

placing  the  surface  layers.  The  embankment  should 
be  allowed  to  settle  and  become  thoroughly  compacted 
before  the  broken  stone  is  placed  upon  it,  and  it  is 
desirable  with  new  embankments  that  they  be  used  for 
a  short  time  by  the  traffic  upon  the  earth  surface  be- 
fore finishing  the  road ;  where,  however,  the  material 
is  well  compacted  in  construction  and  can  be  thor- 
oughly rolled  this  is  not  necessary. 

In  constructing  the  road-bed  its  proper  drainage 
must  be  considered,  and  where  necessary  to  prevent  its 
becoming  wet  under  the  broken  stone  some  means 
should  be  adopted  to  artificially  drain  it. 

Upon  the  completion  of  the  road-bed,  a  layer  of 
broken  stone,  usually  from  3  to  5  inches  in  thickness,  is 
placed  upon  it  and  thoroughly  rolled.  Upon  this  a 
second  layer  is  placed  and  likewise  rolled  to  an  uniform 
surface.  Sometimes  a  third  layer  is  added,  or  in  case 
of  a  very  thin  road  it  may  consist  of  a  single  layer,  the 
number  of  layers  depending  upon  the  thickness  of  the 
road.  When  no  roller  is  used,  the  stone  is  usually 
spread  on  the  surface  of  the  road-bed  to  the  full  thick- 


BROKEN-STONE   ROADS.  9! 

ness  desired  for  the  road,  and  left  to  the  action  of  the 
traffic. 

The  upper  layer  constitutes  the  wearing  surface  of 
the  road,  and  upon  this  it  is  usually  necessary  to  place 
a  thin  layer  of  finer  material  called  binding  material, 
which  may  consist  of  rock  chips,  sand,  small  gravel,  or 
sometimes  loam,  and  is  washed  and  rolled  into  the  inter- 
stices of  the  rock,  with  the  object  of  forming  a  com- 
pact and  impervious  surface.  Binding  material  is  in 
like  manner  often  added  to  the  lower  layers  of  the 
road,  although  this  has  not  been  common  practice. 
The  object  should  be  to  fill  the  voids  in  the  rock  as 
completely  as  possible,  serving  to  make  the  road  one 
solid  mass,  to  bind  the  rock  more  firmly  together,  and 
to  prevent  the  percolation  of  water  through  the  surface. 

When  a  road  is  to  be  constructed  over  a  heavy  soil 
not  easily  drained  and  apt  to  be  wet  and  soft,  a  foun- 
dation consisting  of  a  thin  layer  of  sand  or  gravel  may 
frequently  be  employed  to  advantage.  This  founda- 
tion layer  will  serve  to  prevent  the  stones  of  the  lower 
stratum  of  macadam  from  being  forced  downward  into 
the  soft  material  of  the  road-bed,  or  the  material  of  the 
road-bed  from  forcing  upward  into  the  interstices  of 
the  broken  stone.  This  foundation  may  consist  of  a 
layer  of  sand  or  gravel  from  2  to  5  inches  thick,  and 
should  be  well  compacted  by  rolling  before  the  placing 
of  the  broken  stone. 

ART.  35.    TELFORD  FOUNDATIONS. 

The  distinguishing  feature  of  a  Telford  road  is  its 
paved  foundation.  It  consists  essentially  of  a  pave- 
ment of  stone  blocks  set  upon  the  road-bed  and  cov- 
ered with  one  or  more  layers  of  broken  stone. 


92        A    TEXT-BOOK   ON    ROADS   AND   PAVEMENTS. 

In  forming  a  Telford  road  the  road-bed  is  con- 
structed in  the  same  manner  as  for  macadam,  being 
made  either  level  or  crowned.  A  pavement  is  then 
placed  upon  the  road-bed  from  5  to  8  inches  thick,  de- 
pending upon  the  thickness  to  be  given  the  road 
material,  the  general  practice  being  to  make  the  pave- 
ment about  two  thirds  of  the  total  thickness  of  the 
road.  The  stones  used  for  the  pavement  may  vary 
from  2  to  4  inches  in  thickness  and  8  to  12  inches  in 
length  ;  they  are  set  upon  their  widest  edges  and  with 
their  greatest  lengths  across  the  road.  The  irregulari- 
ties of  the  upper  part  of  the  pavement  are  then  broken 
off  with  a  hammer,  and  all  the  interstices  filled  with 
stone  chips  and  wedged  with  a  light  hammer  so  as  to 
form  a  completed  pavement  of  about  the  thickness  re- 
quired. 

Upon  this  pavement  the  layers  of  broken  stone  are 
placed,  and  the  road-surface  completed  in  the  same 
manner  as  for  a  Macadam  road. 

The  practice  of  Telford  was  to  grade  the  road-bed 
flat,  and  then  construct  his  pavement  deeper  in  the 
middle  than  at  the  sides,  using  for  a  roadway  16  feet 
wide  stones  about  8  inches  deep  at  the  middle  and  5 
inches  at  the  sides.  This  practice  is  still  followed  by 
some  engineers,  but  it  is  now  more  common  and  usually 
considered  preferable  to  make  the  surface,  of  the  road- 
bed parallel  to  the  finished  surface  and  the  pavement 
of  uniform  thickness.  Fig.  19  shows  a  section  of  Tel- 
ford road  as  now  commonly  constructed. 

Some  engineers  in  constructing  Telford  foundations 

'do  not  roll  the  road-bed,  but  simply  bring  it  to  grade, 

and  then  lay  the  pavement  by  bedding  the  stones  in 

the  surface  of  the  road-bed  sufficiently  to  bring  their 


BROKEN-STONE   ROADS.  93 

tops  to  the  proper  height,  in  which  case  it  is  unneces- 
sary to  trim  off  the  tops  with  the  hammer  as  in  the 
common  practice. 

An  objection  sometimes  urged  against  the  Telford 
foundation  is  that  if  the  foundation  be  of  hard  stone  it 
will  cause  the  material  above  to  be  crushed  by  the  loads 
which  come  upon  it,  and  that  greater  durability  in  the 
wear  of  the  road  metal  will  be  obtained  by  having  a 


FIG.  19. 

more  yielding  foundation.  The  durability  of  the  Tel- 
ford  road  has,  however,  been  established  by  long-con- 
tinued usage.  There  is  no  apparent  reason  why  a  firm 
foundation  should  cause  greater  wear  at  the  surface, 
and  the  materials  below  the  surface  are  never  crushed 
in  the  destruction  of  any  broken-stone  road. 

The  relative  value  of  the  two  systems  must  always 
be  determined  by  the  local  conditions  under  which  a 
road  is  to  be  constructed  and  the  necessity  for  such  a 
foundation  in  the  particular  case. 

ART.  36.     CHOICE  OF  FOUNDATION. 

The  proper  foundation  to  be  used  for  a  broken-stone 
road  depends  upon  the  nature  and  condition  of  the 
road-bed  upon  which  it  is  to  be  constructed  and  the 
nature  of  the  traffic  to  pass  over  it.  If  a  firm,  well- 
compacted,  and  thoroughly  drained  road-bed  may  be 
obtained,  of  material  which  will  not  readily  soften 


94        A   TEXT-BOOK    ON    ROADS   AND    PAVEMENTS. 

under  the  action  of  moisture,  there  will  usually  be  no 
need  for  a  special  foundation,  but  the  first  layer  of  the 
macadam  may  be  placed  directly  upon  the  surface  of 
the  road-bed.  If,  however,  the  road-bed  is  of  a  ma- 
terial retentive  of  moisture,  not  thoroughly  drained, 
and  likely  to  become  soft  in  wet  weather,  and  the 
broken  stone  be  laid  immediately  in  contact  with  it, 
the  stones  of  the  lower  layer  of  macadam  may  be  grad- 
ually worked  down  by  the  weight  of  the  traffic  into  the 
soft  earth,  and  the  soil  at  the  same  time  work  up  into 
the  voids  in  the  stone,  causing  a  gradual  disintegration 
of  the  road.  It  may  thus  also  become  retentive  of 
moisture  and  subject  to  the  disrupting  action  of  frost. 
In  this  case  some  foundation  must  be  provided  which 
is  capable  of  resisting  the  penetrating  action  of  the  soft 
material  of  the  road-bed,  and  of  distributing  the  load 
over  it.  This  may  be  the  Telford  foundation  as  de- 
scribed in  Art.  35,  the  sand  or  gravel  foundation  men- 
tioned in  Art.  34,  or  the  Telford  foundation  upon  a 
layer  of  sand  or  gravel,  depending  upon  the  extent  of 
the  difficulty  to  be  met. 

It  is  not  intended  in  the  above  to  imply  that  the  use 
of  a  foundation  of  this  character  should  take  the  place 
of  proper  drainage.  The  advisability  of  artificial 
drainage  should  always  be  carefully  considered,  and 
where  the  road  is  threatened  by  water  which  may  be 
removed  by  the  construction  of  drains  they  should  be 
used,  but  frequently  thorough  drainage  is  difficult  or 
doubtful,  and  it  is  desirable  to  adopt  heavy  construc- 
tion such  as  the  Telford  foundation  gives. 

iTi  some  instances  it  may  be  possible,  by  drains  under 
the  road  and  substituting  porous  material  immediately 
under  the  broken  stone,  to  use  light  macadam  super- 


BROKE X-STONE   ROADS. 


95 


structure  and  do  away  with  the  necessity  for  the  Tel- 
ford  pavement  in  difficult  soils.  Thus  in  Fig.  20  a  con- 
struction is  shown  applicable  to  wet  and  unstable  soils, 
the  space  over  the  centre-drain  and  under  the  middle 
of  the  macadam  being  filled  with  large  rounded  stones, 
which  secure  drainage  and  form  a  stable  bed  for  the 
broken  stone. 

It  is  commonly  claimed  by  the  advocates  of  the 
Macadam  system  of  construction  that  on  any  well- 
drained  and  well-compacted  road-bed  there  will  be  no 
tendency  on  the  part  of  the  stone  to  work  down  or  of 


FIG.   20. 

the  soil  to  work  up,  and  hence  that  the  Telford  foun- 
dation is  an  unnecessary  expense.  The  difficulty  of 
procuring  a  perfectly  stable  and  reliable  road-bed  in 
many  localities  is,  however,  very  generally  recognized, 
and  Telford  pavements  are  largely  used. 

It  would  undoubtedly  be  an  advantage  in  the  con- 
struction of  any  broken-stone  road,  either  Macadam  or 
Telford,  to  have  a  layer  of  sand  or  gravel  between  the 
road-bed  and  the  pavement,  both  as  assisting  drainage 
and  as  providing  against  unequal  settling  of  the  foun- 
dation of  the  road.  The  application  of  it,  however, 
must  in  any  case  depend  upon  its  cost  and  its  apparent 
necessity.  In  cases  where  drainage  is  difficult  and  the 


96       A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

soils  inclined  to  be  damp  and  soft,  it  may  frequently 
prove  the  simplest  solution  of  the  problem. 

Concrete  foundations  are  often  recommended  for 
broken-stone  roads,  and  would  undoubtedly  be  very 
beneficial  in  most  cases,  but  usually  where  so  expensive 
a  foundation  may  be  employed  a  better  surface  might 
advantageously  be  used  than  broken  stone.  It  may 
sometimes  occur,  however,  that,  in  places  where  the 
foundation  is  difficult  to  maintain,  a  light  bed  of  con- 
crete may  prove  of  great  benefit,  as  forming  a  firm  and 
impervious  base  to  rest  upon  damp  and  unstable  soils. 

ART.  37.     MATERIALS  EMPLOYED. 

A  stone  to  be  durable  in  the  surface  of  a  road  should 
be  as  hard  and  tough  as  possible.  The  qualities  of 
toughness  and  resistance  to  abrasion  are  of  more  im- 
portance than  hardness  and  resistance  to  crushing.  A 
stone  may  be  hard  and  brittle,  and  quickly  pound  to 
pieces  in  a  road  surface,  or  it-may  have  a  high  crushing 
strength  and  grind  away  quickly  under  abrasion,  as  is 
the  case  with  some  varieties  of  sandstone.  If,  how- 
ever, it  be  too  soft,  it  may  crush  under  the  loads  com- 
ing upon  it,  and  thus  lack  in  durability. 

A  stone  for  a  road-surface  must  also  resist  well  the 
disintegrating  influences  of  the  atmosphere.  It  should 
be  as  little  absorptive  of  moisture  as  possible  in  order 
that  it  may  not  be  liable  to  injury  from  the  action  of 
frost.  Many  limestones  are  objectionable  on  this  ac- 
count. 

Basalt  and  syenite  are,  in  general,  the  best  materials 
for  this  purpose.  The  harder  limestones  in  some  lo- 
calities make  a  good  and  durable  surface.  Soft  lime- 


BROKEN-STONE   ROADS.  97 

stones  crush  under  the  action  of  the  wheels,  and  soon 
become  dust  and  mud.  Sandstones  as  a  general  thing 
are  not  fit  for  this  use. 

The  material  of  a  road  surface  should  also  be  uni- 
form in  quality;  otherwise  the  wear  of  the  surface  will 
not  be  even,  and  depressions  will  appear  where  the 
softer  material  has  been  placed. 

As  the  under  parts  of  the  road  are  not  subject  to  the 
wear  of  the  traffic,  and  have  only  the  weight  of  the 
loads  to  sustain,  it  is  evidently  not  important  that  the 
foundation  or  lower  layers  be  of  so  hard  or  tough  a 
material  as  the  surface ;  and  hence  it  is  frequently  pos- 
sible, by  using  an  inferior  stone  for  that  portion  of  the 
work,  to  greatly  reduce  the  cost  of  construction. 

A  judgment  as  to  the  value  of  any  given  stone  for 
road  use  can  ordinarily  be  formed  from  what  may  be 
known  of  its  behavior  under  other  uses  to  which  it  may 
have  been  subjected,  or  from  its  appearance  where  it 
has  long  been  exposed  to  the  weather,  together  with 
such  physical  tests  as  may  be  necessary  to  determine  if 
it  possesses  the  special  properties  desired. 

Tests  may  be  made  of  the  power  of  absorption  by 
drying  a  sample,  weighing  it,  then  placing  it  in  water 
and  reweighing  occasionally  until  it  ceases  to  gain  in 
weight.  The  absorption  may  then  be  expressed  as  a 
percentage  of  the  dry  weight.  The  resistance  to  abra- 
sion may  be  found  either  by  grinding  a  sample  upon  a 
polishing-disk,  or  by  rattling  blocks  of  the  material  in 
an  abrasion-cylinder  with  pieces  of  iron,  and  noting 
the  loss  of  weight  in  each  case,  and  comparing  such 
losses  with  those  of  a  stone  of  known  value  under  like 
conditions,  These  examinations  with  tests  of  the 
crushing  strength  of  the  material  will  enable  an  ap- 


98       A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

proximate  idea  to  be  formed  as  to  the  probable  wear- 
ing qualities  of  the  stone. 

The  best  test  of  enduring  properties  will  always  be  a 
knowledge  of  the  stone  as  used  for  any  purpose  where 
it  undergoes  exposure. 

The  selection  of  a  stone  for  road  construction  will  of 
course  always  depend  largely  upon  what  is  to  be  ob- 
tained in  the  locality  of  the  work.  The  importance  of 
a  thoroughly  good  material  in  the  road  surface  is,  how- 
ever, so  great  in  its  effect  upon  the  durability  and  cost 
of  repairs  of  the  road  that  it  may  frequently  be  found 
economical,  on  roads  subjected  to  a  considerable  traffic, 
to  bring  a  good  material  a  considerable  distance  rather 
than  to  use  an  inferior  one  from  the  immediate  vicinity. 
It  may  also  be  suggested  in  this  connection  that  in 
many  instances  railway  transportation  over  a  consider- 
able distance  may  be  small  compared  with  wagon 
transportation  over  a  ::hort  distance,  and  the  impor- 
tation of  good  material  may  add  but  slightly  to  the 
aggregate  cost  of  the  work. 

The  size  to  which  stone  should  be  broken  for  road 
material  depends  to  some  extent  upon  the  nature  of 
the  material.  The  harder  and  tougher  it  is  the  smaller 
the  pieces  may  be  without  danger  of  crushing  or  shat- 
tering under  the  loads  and  shocks  received  in  the  road 
surface,  and  the  smaller  also  they  will  need  to  be  in 
order  to  be  thoroughly  compacted  in  the  road. 

It  is  a  general  custom  to  use  larger  stones  in  the 
bottom  courses  of  a  road  than  at  the  top.  A  rule  very 
commonly  given  is  that  the  stones  for  the  lower  layers 
should  be  at  least  2  inches  in  their  greatest  diameter, 
and  not  more  than  3  inches,  and  that  for  the  surface 


BROKEN-STONE   ROADS. 

layer  the  stones  shall  not   be  greater 
in  greatest  dimension. 

If  of  very  hard  rock  the  surface  layer  may 
inches  as  an  upper  limit  of  size. 

The  size  of  the  rock  in  the  lower  layers  does  not 
seem  of  so  great  importance  as  that  for  the  surface 
layers,  as  it  is  not  directly  subject  to  the  weight  or  the 
abrading  action  of  the  concentrated  wheel-loads,  and  it 
is  probable  that  in  some  cases  unnecessary  expense  is 
incurred  in  following  the  refinements  of  rigid  specifica- 
tions in  this  particular. 

There  is  a  difference  of  opinion  also  among  road- 
builders  as  to  the  advisability  of  using  stone  of  uniform 
size.  Some  insist  quite  strenuously  upon  this  point, 
and  carefully  screen  their  stone  with  the  object  of  get- 
ting it  as  uniform  as  possible  ;  while  others  declare  that 
the  variation  of  size  is  an  advantage,  and  even  that  the 
stone  should  not  be  screened  after  coming  from  the 
crusher,  except  to  remove  the  stone  above  the  limiting 
size  and  when  necessary  to  get  rid  of  foreign  matter 
in  case  it  should  contain  clay  or  earth. 

Uniformity  of  size  probably  makes  the  wear  more 
even,  but  the  presence  of  smaller  fragments  facilitate 
the  binding  together  of  the  material.  The  best  prac- 
tice seems  to  favor  the  exclusion  of  the  fine  material 
from  the  stone,  without  insisting  on  too  great  uniform- 
ity in  size  (stones  being  allowed  probably  from  J  inch 
to  i£  or  2  inches  in  dimension),  and  then  adding  small 
material  after  the  placing  of  the  stone  upon  the  road 
to  assist  binding. 

This  eliminates  the  danger  of  having  portions  of  the 
road  composed  entirely  of  fine  material. 

Road    stone    may  be    broken  by  hand  or  machine. 


IOO     A  TEXT-BOOK   ON   ROADS  AND   PAVEiMENTS. 

Hand-broken  stone  is  usually  preferred  as  cleaner  and 
better  in  shape  for  compacting  in  the  road.  In  England 
hand-breaking  is  largely  practised,  and  it  is  frequently 
asserted  that  machine-breaking  injures  the  stone  by 
crushing  it  in  the  jaws  of  the  machine  with  the  effect 
of  decreasing  its  durability  in  the  road  surface. 

In  American  practice  machine-breaking  is  almost 
exclusively  used.  It  gives  satisfactory  results,  both  as 
to  binding  and  durability,  and  has  the  advantage  of 
greatly  lessening  the  cost  of  construction. 

Gravel  is  frequently  used  for  roads  constructed  in 
the  same  manner  as  with  broken  stone,  both  with  and 
without  the  Telford  foundation.  The  requirements  of 
a  good  gravel  for  this  purpose  are  the  same  as  for  a 
good  stone.  The  stones  of  the  gravel  should  be  sharp 
and  angular,  and  must  possess  the  qualities  of  hardness 
and  toughness.  Water-worn  material  is  therefore  ob- 
jectionable, as  it  will  not  compact  without  the  use  of 
large  amounts  of  soft  binding  material.  In  many  places 
a  hard  flint  gravel  occurs  which  is  not  inferior  to  the 
best  broken  stone. 

Gravel  not  fit  for  surface  material  has  sometimes  been 
used  to  advantage  under  a  surface  layer  of  hard  rock, 
and  in  some  cases  a  surface  of  flint  gravel  has  been 
used  upon  bottom  layers  of  soft  rock. 

Gravel  should  be  screened  to  remove  the  larger 
stones  and  the  fine  material,  and  then  treated  in  the 
same  manner  as  broken  stone. 

Blast-furnace  slag  may  also  in  some  localities  be  used 
as  a  road  material  with  good  results.  Slag  varies 
greatly  in  its  properties,  some  being  porous  and  brittle, 
other  hard  and  tough.  The  ordinary  slag  is  usually  a 
good  material  for  foundation  and  lower  layers  of  tlu: 


BROKEN-STONE   ROADS.  IOI 

road  ;  and  where  a  good,  tough  slag  can  be  obtained  it 
may  also  be  used  for  the  surface.  In  some  places  slag 
is  toughened  for  such  use  by  being  cooled  slowly  under 
a  cover  of  ashes  or  cinders  and  afterward  broken  like 
rock. 

Ashes  or  cinders  may  also  sometimes  be  employed 
as  a  foundation  for  a  thin-broken  stone  road.  They 
serve  to  secure  good  drainage  and  become  very  com- 
pact. In  many  cases  a  considerable  improvement 
might  be  made  in  village  streets  by  substituting  a  layer 
of  ashes  for  an  earth-road  surface,  and  later  a  stone 
surface  could  be  applied  directly  to  the  road  so 
formed. 

ART.  38.     BINDING  MATERIAL. 

It  was  the  practice  of  McAdam  to  require  that  all 
the  stone  used  upon  his  roads  should  be  as  nearly  as 
possible  of  a  uniform  size,  and  that  no  foreign  sub- 
stance be  mixed  with  it.  In  more  recent  practice  it 
has  been  found  advantageous  to  use  a  certain  amount 
of  finer  material  to  fill  the  interstices  between  the 
stones,  and  thus  aid  in  the  compacting  of  the  road  as 
well  as  render  it  less  pervious. 

There  is  considerable  difference  of  opinion  upon  this 
point  among  road-builders.  A  few  still  advocate  the 
system  of  McAdam.  Others  place  a  thin  layer  of 
binding  material  upon  the  surface  of  the  road  and 
work  it  into  the  surface  voids,  while  still  others  dis- 
tribute the  binding  material  through  the  entire  mass  of 
stone  composing  the  road. 

It  is  agreed  that  an  impervious  surface  cannot  be 
formed  of  blocks  of  hard  broken  stone  without  the  ad- 
dition of  some  small  material  to  fill  the  voids.  It  has 


IO2     A  TEXT-BOOK    ON    ROADS   AND   PAVEMENTS. 

also  been  found  that  when  the  rock  is  hard,  such  as  is 
needed  for  good  wear  in  a  road  surface,  it  will  compact 
with  difficulty,  and  that  a  certain  amount  of  binding 
material  is  necessary  in  order  that  the  road  may  be 
brought  to  a  surface. 

The  stone  forming  the  body  of  the  road  should  be 
placed  and  partially  compacted  before  the  addition  of 
the  small  material,  which  may  then  be  worked  into  the 
spaces  between  them. 

The  office  of  the  binding  material  is  to  hold  the 
stones  in  place  and  form  a  bearing  for  them,  as  well  as 
to  prevent  the  passage  of  water  between  them.  It  has 
no  duty  to  perform  in  sustaining  the  loads.  This  is 
the  objection  to  having  the  binding  material  mixed 
with  the  stones  in  advance,  as  would  be  the  case  when 
unscreened  stone  is  used.  A  portion  of  the  road  stones 
would  be  replaced  by  small  material  instead  of  having 
this  material  only  in  such  voids  as  necessarily  exist  be- 
tween the  stones. 

The  quantity  of  binding  to  be  used  is  that  which 
will  be  barely  sufficient  to  fill  all  the  voids  in  the  larger 
material.  It  has  been  contended  that  the  lower  por- 
tion of  the  road  should  be  porous  in  order  to  facilitate 
the  escape  of  any  water  that  may  find  its  way  through 
the  surface,  but  the  tendency  of  the  best  modern  prac- 
tice is  in  the  direction  of  filling  all  the  voids  as  nearly 
as  possible,  thus  making  the  entire  road  practically  one 
solid  body  and  it  is  now  commonly  agreed  that  the  sur- 
face of  a  properly  constructed  broken-stone  road  is  very 
nearly  impervious  to  water. 

The  voids  in  loose  broken  stone  comprise  about  40 
to  50  per  cent  of  the  volume.  In  the  stone  when 
compacted  in  the  road  the  voids  are  somewhat  reduced, 


BROKEX-STONE   ROADS.  IO3 

probably  ranging  from  30  to  40  per  cent  of  the  volume. 
The  voids  may  be  approximately  determined  in  any 
case  by  filling  a  measure  with  the  stone,  shaken  down 
as  closely  as  possible,  and  then  measuring  the  quantity 
of  sand  that  can  be  added  in  the  same  manner. 

Binding  material  may  consist  of  the  screenings  from 
the  broken  stone  used  in  the  road,  of  sand  or  small 
gravel,  or  of  loam.  To  produce  the  best  results  in 
wear  the  material  used  for  binding  should  be  of  equal 
hardness  with  the  road  stone.  Sand,  or  sand  mixed 
with  screenings,  often  gives  satisfactory  results,  and 
is  more  easily  compacted  than  screenings  alone.  With 
the  use  of  loam  it  is  much  easier  to  compact  the  road 
to  a  satisfactory  surface  than  with  harder  material,  es- 
pecially if  the  roller  used  be  a  light  one.  Loam  has 
been  used  in  some  instances  with  satisfactory  results, 
but  the  wisdom  of  its  use  is  questioned  by  most 
authorities. 

ART.  39.    COMPACTING  THE  ROAD. 

The  materials  may  be  compected  in  a  road  either  by 
placing  them  in  position  and  allowing  the  traffic  to 
pass  over  them  or  by  rolling  with  a  steam  or  horse 
roller.  t 

The  first  method  by  itself  is  seldom  practised  when 
it  is  possible  to  avoid  it.  It  is  hard  upon  the  traffic, 
takes  a  long  time  to  reduce  the  road  to  a  compact  con- 
dition, and  a  smooth  surface  is  with  difficulty  pro- 
duced. Where  heavy  horse-rollers  are  employed  they 
are  clumsy  and  inconvenient  to  handle,  and  the  work 
of  rolling  is  slow  as  compared  with  the  steam-roller. 
In  many  instances,  however,  good  results  are  obtained 


104     A   TEXT-BOOK    ON    ROADS   AND   PAVEMENTS. 

with  them.  They  are  not  so  expensive  in  first  cost  as 
steam-rollers,  and  have  not  the  disadvantage  of  fright- 
ening horses. 

Some  road-builders  prefer  light-horse  rollers  of  two 
or  three  tons  weight,  using  them  either  by  themselves 
or  in  connection  with  the  travel.  In  the  latter  case  the 
roller  simply  smooths  off  the  surface  as  the  traffic  does 
the  compacting.  (See  paper  by  James  Owens,  in  Trans- 
actions American  Society  of  Civil  Engineers,  Dec. 
1892.)  These  rollers  are  said  to  work  satisfactorily 
where  soft  binding  material  is  used,  although  longer 
time  is  necessary  than  with  heavier  ones. 

Horse-rollers  are  usually  arranged  so  that  the  direc- 
tion of  motion  may  be  reversed  without  turning  the 
roller  itself  around,  and  also  so  that  the  weight  may  be 
changed  by  placing  additional  weight  inside  the  roller 
or  removing  it. 

Steam  rollers  weighing  from  8  to  15  tons  are  most 
commonly  employed  for  compacting  the  road  materi- 
als. They  have  the  advantage  of  forcing  the  materials 
at  once  into  a  firm  and  compact  mass  and  producing  a 
smooth  surface  for  the  immediate  use  of  travel.  They 
admit  also  of  the  use  of  hard  materials  for  binding. 

In  constructing  a  road  with  the  use  of  a  steam-roller, 
the  road-stone  is  first  put  on  to  the  required  thickness 
and  the  roller  passed  over  it  to  settle  the  stones  into 
place  and  reduce  the  voids  as  much  as  possible.  The 
binding  material,  representing  a  volume  about  equal  to 
the  voids  in  the  stone,  is  then  added,  sprinkled,  and 
rolled  until  the  small  material  is  washed  and  forced  into 
the  interstices,  giving  a  smooth,  hard  surface.  This  is 
repeated  for  each  layer  of  stone,  or  in  some  cases  the 
small  material  is  only  applied  to  the  top  layer. 


BROKEN-STONE    ROADS.  1 05 

A  thin  coating  of  the  binding  material  is  then  spread 
upon  the  surface  and  the  road  thrown  open  for  travel. 

ART.  40.    THICKNESS  OF  ROAD-COVERING. 

The  thickness  nessary  for  a  road-covering  depends 
upon  the  amount  of  the  traffic  it  is  to  bear  and  upon  the 
nature  of  the  foundation  afforded  by  the  road-bed. 
Under  a  heavy  traffic  it  is  advisable  to  make  the  road 
covering  heavier  than  might  be  allowable  for  lighter 
traffic,  in  order  to  provide  for  wear  and  lessen  cost  of 
renewals. 

When  the  road-bed  is  firm,  well  drained,  and  not 
likely  to  soften  at  a  wet  season,  it  will  always  afford  a 
firm  bearing,  upon  which  the  covering  may  rest.  The 
loads  coming  upon  the  road  are  then  simply  transmitted 
through  the  covering  to  the  road-bed  beneath,  and 
there  is  no  tendency  on  the  part  of  the  loads  to  break 
through  the  covering  other  than  by  direct  crushing  of 
its  material.  If,  however,  the  road-bed  may  become 
soft  in  wet  weather,  it  will  then  lose  its  power  to  firmly 
sustain  the  covering  at  all  points,  and  ihe  covering 
must  possess  sufficient  strength  to  bridge  over  places 
where  it  is  not  supported  from  beneath,  or  a  load  com- 
ing upon  it  may  break  through  by  bending  it  down- 
ward at  such  point.  The  thickness  of  road  covering, 
therefore,  must  be  greater  where  the  road-bed  is  less 
perfect. 

The  intensity  of  freezing  that  may  be  expected  also 
has  an  influence  upon  the  necessary  thickness  of  the 
road-covering.  The  effect  of  frost  upon  the  road  will 
depend  in  large  measure  upon  the  condition  of  the 
road-bed,  and  thus  make  the  thickness  depend  in  still 
greater  measure  upon  its  nature.  Freezing  will  not 


IO6     A   TEXT-BOOK   ON    ROADS   AND    PAVEMENTS. 

injure  a  dry  road-bed,  but  if  it  be  damp  and  have  but 
a  thin  covering  the  road  is  likely  to  blow  or  be  thrown 
up  by  the  action  of  frost. 

For  roads  on  considerable  grades  the  thickness  of  the 
road-covering  is  often  reduced  below  what  is  used  on 
flat  ones,  because  of  the  better  drainage  afforded  by 
the  slopes.  It  is  to  be  remarked,  however,  that  if  the 
slopes  are  very  steep  the  wear  of  the  surface  becomes 
so  great,  due  to  the  horses'  efforts  to  obtain  foothold 
and  to  the  washing  of  surface-waters  during  rains,  that 
the  thickness  of  the  coating  should  be  increased. 

Macadam  roads  are  commonly  made  from  4  to  12 
inches  thick,  and  Telford  roads  from  8  to  12  inches,  of 
which  5  to  8  inches  may  be  foundation  pavement. 

A  covering  8  to  10  inches  thick  is  usually  considered 
ample  for  nearly  any  case  of  a  country  road,  unless 
laid  upon  bad  foundation.  In  case  of  a  slope  of  more 
than  3  or  4  degrees  this  may  perhaps  be  reduced  to  6 
inches,  or  with  an  especially  good  road-bed  and  good 
drainage  it  may  even  be  made  4  inches. 

A  thin  road  to  be  effective  must  have  its  interstices 
well  filled  with  binding  material  and  be  thoroughly 
compacted  by  rolling.  It  will  then  present  no  voids  to 
be  filled  by  the  soil  pressing  upward  from  below,  and 
at  the  came  time  it  will  be  practically  impervious  and 
prevent  surface-water  from  reaching  the  road-bed,  thus 
keeping  the  material  in  good  condition  to  sustain  the 
loads.  The  4-inch  roads  of  Bridgeport,  Conn.,  which 
are  often  cited  as  examples  of  successful  work,  are  con- 
structed in  this  manner  of  exceptionally  good  mate- 
rial. In  other  cases  where  thin  roads  have  proved  fail- 
ures the  trouble  may  often  be  traced  to  dampness  in 
the  subsoil  or  to  lack  of  thorough  construction. 


BROKEN-STONE   ROADS.  IO? 

In  many  cases  the  problem  to  decide,  in  determining 
the  thickness  of  a  covering,  is  whether  to  use  heavy 
construction  or  thorough  drainage.  It  is  easier  to  get 
good  results  with  thick  road-coverings,  and  they  are  in 
general  safer  to  use;  but  skilful  adaptation  of  less  ma- 
terial may  often  save  expense  in  construction  with  good 
results.  The  peculiar  conditions  of  each  case  must  de- 
cide what  is  best  for  that  case. 

ART.  41.    CROSS-SECTION. 

The  side-slopes  necessary  to  enable  the  water  which 
falls  upon  a  broken-stone  surface  to  drain  off  readily  to 
the  gutters  is  about  I  in  30  ;  hence  a  crown  should  be 
given  the  road  that  will  permit  that  slope.  In  con- 
struction the  crown  should  be  made  an  inch  or  two 
inches,  depending  upon  the  width  and  thickness  of  the 
road,  higher  than  it  is  intended  to  remain,  in  order  to 
allow  for  settlement  and  wear  under  the  traffic. 

The  arrangement  of  a  cross-section  has  been  referred 
to  in  Art.  30,  and  sections  are  shown  in  Figs.  4,  5,  7,  17, 
and  20.  In  some  cases  on  country  roads  only  the 
middle  portion  of  the  road  is  surfaced  with  broken 
stone,  and  a  roadway  of  earth  is  left  on  each  side,  be- 
tween the  broken  stone  and  the  ditches,  upon  which 
tearris  may  drive  in  dry  weather.  Such  a  roadway 
would  commonly  be  preferred  by  teams  when  the  earth 
is  dry  and  hard,  but  it  renders  the  road  more  expensive 
to  maintain. 

ART.  42.     MAINTENANCE  OF  BROKEN-STONE  ROADS. 

To  maintain  a  brokejvst^iie^road  in  good  condition 
it  is  necessary  first^^^t£aT~ir^p^f^equently  cleaned 


Yf 


108     A   TEXT-BOOK   ON    ROADS   AND   PAVEMENTS. 

of  mud  and  dust,  and  that  the  gutters  and  surface 
drains  be  kept  open  to  insure  the  prompt  discharge  of 
all  water  that  may  come  upon  the  surface  of  the  road. 

The  best  method  of  making  repairs  that  may  become 
necessary  to  the  road-surface  depends  upon  the  char- 
acter of  the  material  composing  the  surface  and  the 
weight  of  the  traffic  passing  over  it. 

If  the  road  metal  be  of  soft  material  which  wears 
easily,  it  will  require  constant  supervision  and  small  re- 
pairs whenever  a  rut  or  depression  may  appear.  Ma- 
terial of  this  kind  binds  readily  with  new  material  that 
may  be  added,  and  may  in  this  manner  frequently  be 
kept  in  good  condition  without  great  difficulty,  while  if 
not  attended  to  at  once  when  wear  begins  to  show  it 
will  very  rapidly  increase,  to  the  great  detriment  of  the 
road.  In  making  repairs  by  this  method,  the  material 
is  commonly  placed  a  little  at  a  time  and  compacted  by 
the  traffic.  The  material  used  for  this  purpose  should 
be  the  same  as  that  of  the  road-surface,  and  not  fine 
material  which  would  soon  reduce  to  powder  under  the 
loads  which  come  upon  it.  By  careful  attention  to 
minute  repairs  in  this  manner  a  surface  may  be  kept 
in  good  condition  until  it  wears  so  thin  as  to  require 
renewal. 

In  case  the  road  be  of  harder  material  that  will  not 
so  readily  combine  when  a  thin  coating  is  added,  the 
repairs  may  not  be  so  frequent,  as  the  surface  will  not 
wear  so  rapidly  and  immediate  attention  is  not  so  im- 
portant. It  is  usually  more  satisfactory  in  this  case  to 
make  more  extensive  repairs  at  one  time,  ?,s  a  larger 
quantity  of  material  added  at  once  may  be  more 
readily  compacted  to  a  uniform  surface,  the  repairs 
taking  the  form  of  an  additional  layer  upon  the  road. 


BROKEN-STONE   ROADS.  1 09 

Where  the  material  of  the  road-surface  is  very  hard 
and  Durable,  a  well-constructed  road  may  wear  quite 
evenly  and  require  very  little,  if  anything,  in  the  way 
of  ordinary  small  repairs  until  worn  out.  It  is  now 
usually  considered  the  best  practice  to  leave  such  a 
road  to  itself  until  it  wears  very  thin,  and  then  renew  it 
by  an  entirely  new  layer  of  broken  stone  placed  in  the 
same  manner  as  in  original  construction,  on  top  of  the 
worn  surface,  and  without  in  any  way  disturbing  that 
surface. 

If  a  thin  layer  only  of  material  is  to  be  added  at  one 
time,  in  order  that  it  may  unite  firmly  with  the  upper 
layer  of  the  road  it  is  usually  necessary  to  break  the 
bond  of  the  surface  material  before  placing  the  new 
layer,  either  by  picking  it  up  by  hand  or,  if  a  steam 
roller  is  in  use,  by  means  of  short  spikes  in  its  surface. 
Care  should  be  taken  in  doing  this,  however,  that  only 
the  surface  layer  be  loosened,  and  that  the  solidity  of 
the  body  of  the  road  be  not  disturbed,  as  might  be  the 
case  if  the  spikes  are  too  long. 


CHAPTER  VI. 
FOUNDATIONS  FOR  PAVEMENTS. 

ART.  43.     PREPARATION  OF  ROAD-BED. 

IN  forming  a  road-bed  upon  which  to  place  a  pave- 
ment, the  earth  should  be  brought  at  subgrade  to  the 
form  of  the  finished  road-surface,  leaving  room  for  the 
superstructure  of  uniform  thickness  to  be  placed  upon 
it.  Thorough  drainage  must  of  course  be  carefully  at- 
tended to  when  necessary.  This  has  been  already  dis- 
cussed in  Chapter  II. 

The  road-bed  after  being  brought  to  the  proper  grade 
should  be  thoroughly  compacted  by  rolling  before 
placing  the  pavement.  Sometimes  in  the  use  of  a 
heavy  roller,  when  the  material  is  of  a  light  nature,  it 
is  shoved  forward  in  a  wave  before  the  roller  and  re- 
fuses to  become  compacted,  in  which  case  a  thin  layer 
of  gravel  or  small  stone  placed  upon  the  surface  of 
earth  before  rolling  may  have  the  effect  of  consolidat- 
ing the  road-bed  under  the  roller  to  a  hard  surface. 

If  the  material  over  which  the  pavement  is  tc  be  con- 
structed is  a  retentive  clay  which  would  become  soft 
when  wet,  it  is  sometimes  desirable  to  excavate  the 
clay  to  a  depth  of  6  inches  or  I  foot  below  the  grade 
of  the  road-bed  and  fill  in  with  sand  or  some  other 
porous  and  non-retentive  material  and  consolidate  this 

no 


FOUNDATIONS   FOR   PAVEMENTS.  Ill 

to  form  the  road-bed.  This  would  seem  unnecessary 
in  case  a  sand  or  gravel  foundation  is  to  be  used,  and 
its  necessity  in  any  other  case  would  depend  upon  the 
likelihood  of  the  road-bed  becoming  wet,  either  through 
natural  wetness  of  the  soil  or  in  consequence  of  the 
use  of  a  pavement  with  open  joints  or  of  pervious  ma- 
terial. If  the  clay  substructure  can  be  kept  dry  it  will 
sustain  the  loads  which  may  be  carried  by  the  road  and 
need  not  be  replaced. 

In  constructing  a  road-bed  to  bear  a  pavement  the 
same  principles  would  be  involved  as  in  the  earthwork 
of  a  common  road  which  has  been  discussed  in  Art.  25. 

ART.  44.    PURPOSE  OF  FOUNDATION. 

The  chief  object  of  the  foundation  or  base  of  a 
pavement  is  to  distribute  the  concentrated  loads  which 
come  upon  the  surface  of  the  road  over  a  greater  area 
of  the  usually  softer  and  weaker  road-bed,  in  order 
that  these  loads  may  not  produce  indentations  in  the 
surface. 

In  a  foundation  composed  of  independent  blocks 
extending  through  its  thickness,  as  in  the  case  of  a 
stone-block  pavement  in  which  the  blocks  rest  directly 
upon  the  road-bed  or  upon  a  thin  layer  of  sand,  the 
load  which  comes  upon  the  top  of  any  block  will  be 
distributed  over  the  area  covered  by  the  base  of  the 
block. 

Where  the  foundation  is  composed  of  small  indepen- 
dent particles,  like  sand  or  loose  rounded  gravel,  with 
no  cohesion  through  the  mass,  the  pressure  is  distrib- 
uted over  the  base  of  a  cone  whose  vertex  is  in  the 
point  of  application  of  the  load,  and  the  inclination  of 


112     A  TEXT-BOOK   ON   ROADS   AND    PAVEMENTS. 

whose  elements  depends  upon  the  friction  of  the  par- 
ticles of  the  material  upon  each  other.  In  this  case 
the  area  over  which  the  load  is  distributed  varies 
directly  as  the  square  of  the  thickness  of  the  founda- 
tion. Sand,  it  is  to  be  observed,  has  also  the  property, 
when  confined  as  in  a  foundation,  on  account  of  its  in- 
compressible nature,  of  adjusting  itself  to  a  uniform 
pressure  and  resisting  the  deformation  of  the  road-bed. 

If  the  small  pieces  composing  the  foundation  are 
cemented  together,  or  held  as  in  masses  of  angular 
fragments  by  the  interlocking  of  the  angles,  the  foun- 
dation may  act  more  or  less  as  a  whole,  causing  a  distri- 
bution of  the  load  over  a  considerable  area,  the  extent 
of  which  will  depend  upon  the  resistance  of  the  mass 
to  bending. 

The  bases  most  commonly  employed  for  pavements 
are  sand,  broken  stone,  and  concrete.  Foundations  of 
brick  and  wood  are  also  frequently  employed  for  pave- 
ments of  the  same  materials. 


ART.  45.    SAND  FOUNDATIONS. 

Sand  foundations  as  sometimes  used  under  block 
pavements  consist  simply  of  a  bed  of  sand  from  6  to 
12  inches  deep,  spread  over  the  road-bed  and  thor- 
oughly compacted  by  rolling. 

To  obtain  the  best  results  the  sand  should  be  placed 
in  layers  of  3  or  4  inches  depth  and  each  layer  be 
rolled  before  the  addition  of  the  next.  This  insures 
the  equal  consolidation  of  the  entire  foundation.  Un- 
der a  heavy  roller  the  sand  may  usually  be  compressed 
to  j  or  even  f  of  its  original  volume. 


FOUNDATIONS   FOR   PAVEMENTS.  113 

Well-constructed  sand  foundations  are  often  very 
efficient  for  quite  severe  service. 

Where,  as  is  very  common,  pavements  are  set  on  a 
bed  of  loose  sand,  subsequent  settlement  of  the  base 
is  likely  to  take  place,  even  if  the  surface  of  the  pave- 
ment be  well  rammed  during  construction. 

Block  pavements  are  also  frequently  set  upon  a  thin 
layer  of  loose  sand.  This,  however,  can  hardly  be 
considered  a  sand  foundation,  as  the  sand  only  acts  as 
a  cushion  to  protect  the  earth  of  the  road-bed  from 
direct  contact  with  the  blocks.  It  also  at  the  same 
time  may  facilitate  drainage. 

ART.  46.    BASES  OF  GRAVEL  AND  BROKEN  STONE. 

Foundations  of  gravel  and  broken  stone  are  con- 
structed in  much  the  same  manner  as  those  of  sand. 
Small  gravel  will  act  in  much  the  same  manner  as 
sand.  In  general,  however,  as  a  base  for  a  pavement 
composed  of  independent  blocks  these  larger  materials 
are  inferior  to  one  either  of  sand  or  concrete. 

These  foundations  may  also  be  constructed  by  the 
use  of  binding  material  sufficient  to  thoroughly  con- 
solidate the  mass,  after  the  manner  of  broken-stone 
roads,  and  when  carefully  made  are  quite  efficient  in 
use. 

In  Liverpool  a  base  has  been  largely  used  under 
stone-block  pavements,  which  consists  of  broken  stone 
grouted  with  coal  pitch  and  creosote  oil,  then  covered 
with  quarry-chippings  and  thoroughly  rolled.  This  is 
said  to  give  satisfactory  results  on  streets  of  moderate 
traffic,  and  is  cheaper  than  hydraulic  concrete. 


114     A  TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

ART.  47.    CONCRETE  BASES. 

The  best  base  for  general  use  under  pavements  is 
without  doubt  that  formed  of  good  hydraulic  cement 
concrete.  A  bed  of  concrete  made  of  good  hydraulic 
cement,  well  rammed  and  allowed  to  set  and  harden, 
becomes  a  practically  monolithic  structure,  nearly  im- 
pervious to  water  and  possessing  a  high  degree  of 
strength  against  crushing. 

The  concrete  is  formed  of  a  mixture  of  cement,  sand, 
and  broken  stone  or  gravel.  The  proportions  vary  for 
different  work  and  with  the  character  of  the  materials. 
With  good  Portland  cement,  the  most  common  pro- 
portions for  ordinary  work  are  about  one  part  cement, 
three  parts  sand,  and  five  to  seven  parts  broken  stone. 
With  the  various  natural  cements  the  proportions  vary 
from  the  above  to  one  part  cement,  two  parts  sand, 
and  three  of  broken  stone. 

In  preparing  the  concrete  the  cement  and  sand  should 
first  be  thoroughly  mixed  dry,  then  sufficient  water 
added  to  reduce  the  mass  when  well  worked  to  a  suf- 
ficiently plastic  condition  to  be  coherent.  It  should 
not  be  made  soft  or  semi-fluid.  The  amount  of  water 
necessary  to  give  the  proper  consistency  should  be 
first  determined,  and  then  this  quantity  added  each 
time  to  the  mixed  sand  and  cement.  The  mortar 
should  then  be  reduced  to  a  plastic  condition  by  work- 
ing it,  and  not  by  the  addition  of  more  water.  The 
water  should  never  be  applied  to  the  mortar  from  a 
hose. 

When  the  mortar  has  been  well  mixed  it  is  added  to 
the  broken  stone,  which  should  first  be  dampened  by 
sprinkling  sufficiently  to  wet  the  surfaces  and  wash  the 


FOUNDATIONS   FOR   PAVEMENTS.  11$ 

dust  from  them.  The  mass  is  then  to  be  mixed  until 
the  mortar  and  stone  are  uniformly  distributed  through 
it.  This  is  commonly  done  by  turning  the  whole  mass 
over  several  times  with  shovels. 

The  concrete,  when  well  mixed,  is  placed  in  position 
and  rammed.  The  more  thorough  the  ramming  the 
better. 

The  foundation,  after  completion,  is  allowed  to  re- 
main several  days  before  the  pavement  is  placed  upon 
it,  five  or  six  days  are  usually  sufficient,  in  order  that 
the  mortar  may  become  entirely  set.  During  setting 
the  concrete  should  be  protected  from  the  drying  ac- 
tion of  the  sun  and  wind,  and  should  be  kept  damp  to 
prevent  the  formation  of  drying  cracks. 

The  sand  used  for  mortar  should  be  clean  and  sharp. 
The  broken  stone  should  be  limited  to  2%  or  3  inches 
in  largest  dimension,  and  in  some  case  is  limited  to  2 
inches. 

Small  gravel  is  sometimes  used  in  place  of  sand  for 
the  mortar  to  form  concrete,  although  the  smaller  ma- 
terial is  preferable.  Gravel  is  also  sometimes  mixed 
with  the  broken  stone  in  order  that  a  less  quantity  of 
mortar  may  be  necessary  to  fill  the  voids  in  the  stone, 
thus  making  an  impervious  but  weaker  concrete. 

In  some  cases  foundations  are  prepared  by  placing 
the  broken  stone  and  the  mortar  upon  the  road-bed  in 
alternate  layers,  and  mixing  by  ramming  them  into  po- 
sition. A  layer  of  stone  is  first  placed  and  wet,  then  a 
thin  layer  of  mortar  ;  a  second  layer  of  stone  is  added 
and  rammed  into  the  mortar,  after  which  other  lay- 
ers are  placed  and  rammed  in  like  manner.  This 
method  has  been  followed  for  the  best  class  of  work 
in  Liverpool,  and  is  reported  as  giving  good  results. 


Il6     A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

Frequently  in  laying  asphalt  pavements,  a  concrete 
is  used  for  the  base,  which  is  formed  by  mixing 
asphaltic  or  coal  tar  paving  cement  with  the  broken 
stone.  This  is  known  as  a  bituminous  base  and  is 
similar  to  that  mentioned  at  the  end  of  Art.  46.  It  is 
commonly  constructed  by  placing  the  broken  stone 
upon  the  road-bed  to  a  depth  of  about  4  or  5  inches 
and  rolling  it  to  a  firm  and  even  bearing,  as  in  the  con- 
struction of  a  broken  stone  road,  after  which  a  coating 
of  coal-tar  cement  is  given  to  it,  about  one  gallon  of 
the  cement  being  required  for  a  square  yard  of  the 
base. 

ART.  48.    BRICK  FOUNDATIONS. 

Foundations  of  brick  have  frequently  been  used 
under  brick  pavements.  The  pavement  in  such  cases 
consists  of  two  layers  of  brick,  with  sand  between,  and 
is  known  as  double-layer  pavement.  These  foundations 
are  usually  formed  by  placing  upon  the  road-bed  a 
layer  of  sand  or  gravel  3  or  4  inches  thick,  which  is 
rolled  thoroughly  to  a  uniform  surface,  and  then  re- 
ceives a  layer  of  brick,  commonly  laid  flat  and  with 
the  greatest  dimension  lengthwise  of  the  street.  These 
bricks  are  laid  as  closely  as  possible  with  broken  joints. 
The  joints  are  filled  with  sand  carefully  swept  in,  and 
the  bricks  are  rammed  to  a  firm  bearing. 

Upon  this  course  of  brick  is  placed  a  cushion  layer 
of  sand,  and  then  the  surface  layer.  The  bricks  of  the 
lower  layer  may  be  of  a  cheaper  grade  than  the  sur- 
face-paving brick,  as  they  are  not  required  to  resist  the 
attrition  of  travel 


FOUNDATIONS  FOR  PAVEMENTS.  1 1/ 

ART.  49.    SAND  AND  PLANK  FOUNDATIONS. 

Under  many  wood  pavements,  and  sometimes  under 
brick  surfaces,  foundations  formed  of  sand  and  planks 
are  used.  These  foundations  differ  somewhat  in 
construction  in  various  localities,  but  are  essentially  a 
bed  of  sand  or  gravel,  upon  which  is  placed  a  layer  of 
tarred  boards  which  support  the  surface  layer. 

It  is  common  to  use  a  layer  of  sand  3  or  4  inches 
thick,  which  is  compacted  by  rolling,  after  which  the 
boards  are  laid  lengthwise  of  the  street  close  together, 
so  as  to  form  a  floor  upon  which  the  blocks  may  be 
set.  With  a  brick  surface  a  cushion  coat  of  sand  is 
used  under  the  surface  layer. 

Sometimes  two  layers  of  one-inch  tarred  boards  are 
employed,  the  lower  being  laid  crosswise  of  the  street 
and  the  upper  lengthwise  of  it.  In  other  cases,  the 
boards  of  a  single  thickness  are  nailed  to  scantling  laid 
across  the  street  and  bedded  in  the  sand.  The  boards 
must  in  all  cases  press  evenly  upon  the  layer  of  sand 
that  covers  the  road-bed. 

ART.  50.    DEPTH  OF  FOUNDATION. 

The, thickness  required  for  the  foundation  of  a  pave- 
ment depends  upon  the  nature  of  the  soil  upon  which 
it  is  to  rest,  and  upon  the  extent  and  weight  of  the 
travel  to  which  it  is  to  be  subjected. 

On  a  well-drained  road-bed  of  good  material,  a  depth 
of  6  inches  of  concrete  is  usually  sufficient,  even  under 
heavy  loads.  6  or  8  inches  of  well-compacted  sand 
or  gravel  will  likewise  usually  suffice. 

When,  however,  the  road-bed  is  of  a  retentive  ma- 


Il8     A   TEXT-BOOK   ON    ROADS   AND    PAVEMENTS. 

terial  and  likely  to  become  wet  and  soft,  the  founda- 
tion should  possess  sufficient  strength  not  to  be  broken 
through  at  points  where  the  supporting  power  of  the 
road-bed  may  be  destroyed  by  water.  It  must  also  be 
able  to  resist  the  action  of  frost  upon  the  soil  below. 
In  such  cases  12  inches  of  concrete  may  be  necessary, 
or  12  to  1 6  inches  of  sand  or  gravel  may  be  desirable. 

Under  light  traffic  with  good  conditions,  a  less  depth 
may  be  sometimes  used,  4  inches  of  concrete  is  fre- 
quently employed  to  save  expense,  although  6  is  the 
more  common  depth. 

It  is  always  important  that  the  foundation  be  suffi- 
cient. The  yielding  of  the  base  of  the  pavement  means 
its  destruction. 

If  a  firm  and  durable  foundation  be  employed,  the 
surface  may  be  renewed  when  necessary  or  changed 
from  one  material  to  another  without  disturbing  the 
base,  but  if  the  base  be  weak  the  surface  will  be  de- 
stroyed. 

The  saving  of  expense  should  be  at  the  top  rather 
than  at  the  bottom  of  a  pavement. 

It  may  be  here  observed  that  no  definite  prescrip- 
tion for  any  pavement,  either  as  to  choice  of  founda- 
tion or  as  to  methods  of  construction  can  fit  all  cases. 
What  is  most  successful  in  one  case  is  quite  inappli- 
cable in  another.  The  blind  following  of  particular 
rules  by  those  not  conversant  with  the  principles  upon 
which  they  are  based  has  been  the  cause  of  many  fail- 
ures. Judgment  must  always  be  used  in  weighing  the 
local  conditions  of  the  problem  in  hand. 


CHAPTER   VII. 
BRICK  PAVEMENTS. 

ART.  51.     PAVING-BRICK. 

THE  requisites  for  a  good  paving-brick  are  that  it 
shall  be  hard,  tough,  and  impervious,  as  well  as  capable 
of  enduring  against  the  disintegrating  influences  of  the 
weather. 

The  bricks  in  most  common  use  are  made  from  fire- 
clay of  an  inferior  quality,  or  from  an  indurated  clay  or 
shale  of  somewhat  similar  composition. 

These  clays  consist  essentially  of  silicate  of  alumina, 
with  usually  small  percentages  of  lime,  magnesia,  iron, 
potash,  soda,  and  sometimes  other  elements.  The 
range  of  composition  for  clays  in  common  use  is  ap- 
proximately as  follows: 

Silica 60  to  75  per  cent. 

,  Alumina 10  to  25  "  " 

Iron  oxide 3  to    8  "  " 

Lime o  to    4  "  " 

Magnesia o  to    3  "  " 

Potash 0.5  to    3  "  " 

Soda - o  to    2  "  " 

In  a  few  cases  the  quantity  of  lime  is  greater  vary- 
ing from  8  to  12  per  cent. 

no 


120  A  TEXT- BOOK  ON  ROADS  AND  PAVEMENTS. 

When  the  clay  is  very  nearly  pure  silicate  of  alumina, 
it  is  capable  of  withstanding  a  high  degree  of  heat 
without  fusing,  and  is  known  as  fire-clay.  As  the  per- 
centages of  other  ingredients  increase  it  becomes  more 
fusible.  The  lime,  magnesia,  potash,  and  soda  act  as 
fluxing  agents,  and  the  readiness  with  which  the  clay 
can  be  melted  depends  upon  the  relative  quantities  of 
refractory  and  fluxing  materials  that  it  may  contain. 

Silica  in  excess  tends  to  make  the  brick  weak  and 
brittle,  while  too  great  quantity  of  alumina  causes  the 
brick  to  crack  and  warp  in  the  shrinking  which  occurs 
during  burning.  The  proper  adjustment  of  the  rela- 
tions between  these  elements  is  necessary  to  good 
results. 

The  quantity  of  lime  in  the  clay  is  an  important 
matter,  as  the  presence  of  lime  in  an  uncombined  state 
in  the  brick  may  be  productive  of  disintegration  when 
the  brick  is  exposed  to  the  weather.  A  large  percent- 
age of  lime  in  a  clay  is  therefore  to  be  regarded  with 
suspicion,  although  not  necessarily  as  cause  for  con- 
demnation, as  its  effect  depends  upon  the  state  of  com- 
bination of  the  various  ingredients  of  the  brick.  Mag- 
nesia probably  acts  in  much  the  same  manner  as  lime. 
Potash  and  soda  are  considered  desirable  elements  in 
quantities  to  properly  flux  the  clay  in  burning. 

The  fineness  of  a  clay  is  also  a  matter  of  importance, 
both  because  a  fine  clay  will  fuse  at  a  lower  tempera- 
ture than  a  coarse  one,  and  because  fineness  is  neces- 
sary to  the  production  of  even  and  close  grained  brick, 
and  therefore  conduces  to  make  them  tough  and  im- 
pervious. 

To  produce  a  good  paving-brick,  a  clay  is  required 
which  will  vitrify  at  a  high  heat.  A  very  refractory 


BRICK   PAVEMENTS.  121 

clay  will  make  a  porous  brick,  while  if  it  melts  at  too 
low  a  temperature  it  cannot  be  burned  sufficiently  to 
become  hard  and  tough. 

The  methods  of  manufacturing  paving-brick  vary  in 
different  localities  according  to  the  character  of  the 
material  to  be  worked.  They  are  quite  similar  to  those 
in  use  for  common  brick,  only  more  thoroughly  exe- 
cuted. 

The  clay  is  commonly  reduced  to  a  fine  powder, 
tempered  with  water  and  passed  through  a  machine 
that  moulds  the  bricks,  which  are  then  dried  and  after- 
ward burned.  Repressed  bricks  are  those  which  are 
compressed  in  a  mould  after  coming  from  the  brick 
machine  and  before  drying. 

The  process  of  burning  occupies  usually  from  10  to 
15  days. 

The  heat  is  at  first  slowly  applied  to  expel  the  water, 
then  raised  to  a  high  temperature  for  several  days, 
after  which  the  bricks  are  very  slowly  cooled. 

There  is  considerable  difference  of  opinion  among 
engineers  and  manufacturers  as  to  the  exact  amount  of 
burning  necessary.  It  is  usually  stated  that  the  brick 
should  be  burned  to  the  point  of  vitrification,  but  not 
completely  vitrified.  The  burning  must  be  thoroughly 
done  to  produce  a  strong  and  impervious  brick,  but 
there  is  undoubtedly  a  point  beyond  which,  for  some 
brick,  further  burning  causes  brittleness.  Very  gradual 
cooling  is  also  necessary  in  order  to  toughen  the  brick. 

Smoothness  and  uniformity  of  texture  in  a  paving 
brick  is  an  important  consideration  as  affecting  its  re- 
sistance both  to  crushing  and  to  abrasion.  The  broken 
surface  of  the  brick  should  present  a  uniform  appear- 
ance both  in  texture  and  in  color. 


122     A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

All  of  the  bricks  used  in  the  same  pavement  should 
also  be  of  the  same  degree  of  hardness  and  toughness 
in  order  that  the  pavement  may  wear  evenly,  and  to 
this  end  careful  inspection  should  always  be  given  to 
the  bricks  proposed  for  use,  and  all  of  those  which  are 
defective,  soft  from  imperfect  burning,  brittle  from 
everburning  or  quick  cooling,  or  cracked,  should  be 
rejected. 

The  usual  and  most  convenient  size  for  the  paving 
bricks  is  about  the  same  as  that  of  building  bricks. 
These  dimensions,  however,  vary  considerably  in  prac- 
tice, and  scarcely  any  two  manufacturers  make  them 
exactly  of  the  same  size.  The  ordinary  dimensions  are 
from  7£  to  9  inches  long,  2  to  2j  inches  wide,  and  3^ 
to  4^  inches  deep,  requiring  from  50  to  75  bricks  per 
square  yard  when  set  on  edge.  The  corners  are  some- 
times rounded  off  by  a  curve  of  J  to  ^  inch  radius,  or 
bevelled  off,  which  is  not  an  advantage  on  ordinary 
work  where  close  joints  are  desirable.  There  seems  to  be 
no  advantage  to  be  gained  in  making  larger  bricks,  as  has 
been  proposed.  It  is  difficult  to  burn  a  larger  brick,  a 
better  foothold  is  given  by  the  pavement  with  joints 
close  together,  and  if  a  firm  foundation  be  secured  the 
4-inch  depth  is  ample,  while  the  numerous  joints  intro- 
duce no  element  of  weakness. 

ART.  52.    TESTS  FOR  PAVING  BRICK. 

To  determine  the  probable  durability  of  brick  de- 
signed for  use  in  paving,  mechanical  tests  may  be 
applied  which  will  show  the  relative  rank  of  different 
samples  in  their  most  important  characteristics.  The 
tests  ordinarily  proposed  or  used  for  this  purpose  are 


BRICK   PAVEMENTS.  12$ 

those  of  crushing  strength,  transverse  strength,  abra- 
sion and  impact,  absorption,  and  specific  gravity. 

For  the  crushing  tests  it  is  common  to  use  2-inch 
cubes,  sawed  from  the  brick,  and  brought  to  a  surface 
by  grinding,  without  cutting  with  chisels  or  hammer- 
ing the  specimens,  in  order  to  prevent  any  injury  to  the 
material  which  might  reduce  its  power  of  resistance. 
A  sheet  of  soft  paper  or  a  thin  layer  of  plaster  of  Paris 
is  sometimes  used  between  the  ends  of  the  specimen 
and  the  compression  blocks  of  the  testing-machine  to 
equalize  the  pressure  over  the  surface  of  the  cube. 

The  result  of  a  compressive  test  of  stone  or  brick 
depends  very  largely  upon  how  it  is  made,  and  the 
results  of  tests  are  only  properly  comparable  with 
others  made  in  the  same  manner  and  with  equal  care. 
The  use  of  plaster  beds  as  suggested  above,  it  is 
thought,  conduces  greatly  to  regularity  of  result  in  the 
work  of  different  men,  as  it  tends  to  reduce  the  effect 
of  differences  in  the  accuracy  of  dressing  the  surfaces 
of  contact.  The  size  of  the  test-piece  is  also  important, 
the  strength  usually  increasing  as  the  size  increases. 
Small  pieces,  ij  or  2  inch  cubes,  are  usually  employed, 
because  of  the  large  force  necessary  to  crush  a  whole 
brick,  although  where  machinery  exists  capable  of  doing 
it  the  larger  tests  entail  much  less  work  in  preparing 
specimens. 

It  is  to  be  observed  that  the  actual  crushing  strength 
of  a  brick  is  not  a  matter  of  special  importance  in 
so  far  as  any  danger  of  the  crushing  of  the  material 
in  the  pavement  is  concerned,  as  no  stress  can  there 
come  upon  it  under  ordinary  circumstances  which 
would  endanger  even  a  very  weak  specimen  from  direct 
crushing.  It  is  thought,  however,  that  to  some  extent 


124     A  TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

the  value  of  the  brick  is  indicated  by  its  resistance  to 
crushing,  coupled,  of  course,  with  a  proper  examina- 
tion of  its  other  necessary  attributes.  A  brick  which 
possesses  a  high  crushing  strength  is  not  necessarily 
a  good  paving  brick,  as  it  may  at  the  same  time  be 
brittle  or  of  such  composition  as  to  easily  disintegrate 
under  the  action  of  the  weather;  but  one  that  yields  to 
a  low  crushing  strength  is  usually  weak  in  wearing 
qualities  and  not  fit  for  the  purpose.  A  goc  d  paving 
brick,  in  the  form  of  a  2-inch  cube,  will  usually  show  a 
resistance  to  crushing  of  not  less  than  10,000  pounds 
per  square  inch.  Much  higher  values  are  sometimes 
used  in  specifications,  but  their  advantage  is  at  least 
doubtful. 

The  transverse  strength  is  tested  upon  a  whole  brick 
by  supporting  it  on  edge  upon  knife-edges  6  inches  apart, 
and  bringing  the  load  by  a  third  knife-edge  upon  the 
middle  of  the  brick.  Soft  paper,  cloth,  or  leather  may 
be  interposed  between  the  knife-edges  and  the  brick  to 
prevent  the  abrasion  of  the  brick  at  the  points  of  con- 
tact. This  test  is  easier  to  conduct  satisfactorily,  and 
probably  gives,  in  general,  a  more  reliable  indication  of 
the  value  of  the  material  than  the  crushing  strength. 
It  calls  into  play  not  only  the  compressive  but  the  ten- 
sile strength  of  the  brick.  In  the  conduct  of  the  test 
<:are  is,  however,  quite  as  essential  as  in  the  crushing 
test.  It  is  especially  important  that  the  bric£  shall 
have  a  perfectly  even  bearing  upon  the  supports  before 
the  application  of  the  load,  in  order  that  it  may  not  be 
subjected  to  a  twist  under  the  load. 

The  modulus  of   rupture  for  the  material  may  be 

3  Wl 
deduced  by  the  ordinary  formula  R  =  —  r-»t  in  which 


BRICK  PAVEMENTS.  125 

R  is  the  modulus  of  rupture  in  pounds  per  square 
inch,  W\s  the  breaking  load  at  the  centre  in  pounds, 
/  is  the  length,  b  the  breadth,  and  d  the  depth  of  the 
specimen,  all  in  inches. 

The  modulus  of  rupture  of  paving  bricks  of  good 
quality  ranges  from  1500  to  3000  pounds  per  square 
inch. 

The  fracture  of  a  tough  and  homogeneous  specimen 
under  a  transverse  load  should  be  a  clean  break 
through  the  middle  of  the  brick,  and  a  close  observa- 
tion of  the  breaks  may  frequently  be  of  considerable 
assistance  in  forming  an  idea  of  these  qualities, 
although  they  may  not  be  directly  represented  by  the 
load  required  to  break  the  specimen.  The  shattering 
of  the  brick  in  breaking,  or  irregular  breaks  extending 
from  the  point  of  application  of  the  load  to  one  of  the 
points  of  support,  are  apt  to  indicate  brittleness  of  the 
material. 

Tests  for  abrasion  and  impact  have  been  conducted 
in  various  ways,  and  there  is  no  standard  method  by 
which  the  value  of  the  material  to  resist  these  forces 
may  be  quantitatively  expressed.  Each  set  of  experi- 
ments usually  compares  the  various  specimens  by  sub- 
jecting them  all  to  the  same  treatment  at  once,  and  in- 
cluding, as  a  standard,  specimens  of  stone  or  brick 
of  known  value. 

The,  usual  method  of  conducting  this  test  is  to  place 
whole  bricks  in  a  foundry  tumbler  with  a  given  weight 
of  cast-iron,  and  determine  the  loss  of  weight  of  the 
specimens  after  a  certain  number  of  revolutions.  It  is 
customary  to  repeat  this  operation  for  two  or  three  in- 
dependent periods,  usually  about  one  half  hour  each. 
The  loss  during  the  first  period  is  largely  influenced  by 


126     A  TEXT-BOOK   ON   ROADS  AND    PAVEMENTS. 

the  chipping  of  corners  of  the  bricks,  and  the  test  of 
wear  would  be  based  more  upon  the  later  periods.  The 
results  of  such  tests  made  at  different  times  and  places 
are  not  comparable  with  each  other,  but  they  are  useful 
as  showing  the  comparative  merit  of  the  samples  at 
hand  in  each  case. 

Where  specifications  include  a  test  of  this  character, 
it  is  better  to  require  that  the  brick  shall  bear  a  certain 
relation  to  a  standard  material  to  be  included  in  the 
test  rather  than  to  name  a  minimum  percentage  of  loss, 
as  there  is  necessarily  considerable  uncertainty  in  the 
test,  unless  it  can  be  conducted  according  to  a  method 
the  results  of  which  upon  standard  material  are  definitely 
known,  and  the  details  of  which  may  at  any  time  be 
reproduced  both  as  to  the  apparatus  to  be  used  and  as 
to  the  nature,  sizes  and  forms  of  the  abrading  material. 
It  would  be  advantageous  if  certain  standards  could  be 
recognized  and  used  in  such  work  which  would  enable 
comparisons  to  be  made  of  different  material  upon  the 
basis  of  percentages  obtained  by  various  men  in  differ- 
ent localities. 

It  is  also  desirable  to  use  pieces  less  in  size  than  a 
whole  brick,  in  order  that  the  abrasion  during  the  test 
may  not  be  altogether  upon  the  outside  of  the  brick. 

It  is  quite  true  that  the  action  to  which  the  material 
is  subjected  in  a  test  of  this  character  is  quite  different 
from  the  wear  to  which  the  material  is  subjected  when 
firmly  held  in  the  pavement,  but  the  qualities  necessary 
to  resist  wear  in  the  two  cases  are  quite  similar.  We 
may  form  an  idea  of  whether  a  material  is  suitable  for 
the  proposed  use  from  such  experiments,  although  no 
definite  idea  of  the  amount  of  wear  that  it  will  endure 
can  be  obtained  from  them. 


BRICK   PAVEMENTS.  I2/ 

Absorption  tests  are  made  by  weighing  the  specimen 
dry,  then  saturating  it  and  weighing  again,  and  stating 
the  absorption  as  a  percentage  of  the  dry  weight.  In 
making  the  test,  the  brick  is  first  thoroughly  dried  by 
placing  it  in  a  drying  oven  at  a  temperature  of  212° 
Fahr.  until  it  ceases  to  lose  in  weight.  It  is  then  placed 
in  water  and  permitted  to  remain  until  saturated  and 
weighed  again.  In  some  cases  the  brick  is  left  in  the 
water  twenty-four  hours,  in  others  until  it  ceases  to 
materially  gain  in  weight.  The  latter  is  preferable,  as 
the  'absorption  of  various  bricks  may  differ  not  only  in 
amount,  but  also  in  rate.  Whole  bricks  should  not  be 
used  for  absorption  tests,  as  the  outside  is  likely  to  be 
less  absorptive  than  the  interior.  A  good  paving-brick 
will  not  usually  absorb  more  than  5  per  cent  of  water, 
and  many  of  the  best  varieties  will  take  less  than  I  per 
cent. 

Tests  for  the  presence  of  free  lime  may  be  made  by 
placing  a  specimen  in  water  and  leaving  for  a  few  days. 
If  uncombined  lime  be  present  in  considerable  quantity 
it  will  cause  the  brick  to  crack  or  blow  on  the  surface. 
Tests  for  this  purpose  may  also  be  made  by  pulverizing 
a  small  portion  of  the  brick,  washing  it  with  water,  and 
determining  the  percentage  of  soluble  matter  contained 
by  the  brick. 

Such  tests  may  aid  in  forming  a  judgment  as  to  the 
value  of  a  material  for  paving  purposes,  the  only  con- 
clusive test,  however,  is  the  record  of  use  of  the  same 
material  in  similar  work.  If  the  normal  value  of  a  cer- 
tain make  of  brick  be  known,  tests  may  indicate 
whether  a  special  lot  be  of  standard  quality.  They 
cannot  be  conclusive  as  to  the  value  of  an  untried 
material. 

"> 


128     A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  53.    FOUNDATIONS  FOR  BRICK  PAVEMENTS. 

A  brick  pavement  should  have  a  firm  foundation. 
As  the  surface  is  made  up  of  small  independent  blocks, 
each  brick  must  be  adequately  supported  from  below, 
or  the  loads  coming  upon  it  may  force  it  downward 
and  cause  unevenness.  The  wear  of  the  pavement 
depends  very  largely  upon  the  maintenance  of  a  smooth 
even  surface,  as  any  unevenness  will  cause  the  bricks  to 
chip  on  the  edges,  and  also  produce  impact  from  the 
loads  passing  over  the  pavement. 

The  best  foundation  for  a  brick  pavement  is  doubtless 
one  of  concrete,  laid  after  the  manner  given  in  Art.  47. 
For  light  or  moderately  heavy  traffic,  such  as  that  of 
the  ordinary  small  city,  the  concrete  is  usually  placed 
6  inches  thick.  If  the  traffic  be  very  heavy  9  inches 
may  be  necessary,  and  where  from  any  cause  the  road- 
bed is  not  firm  it  may  be  advisable  to  still  farther  in- 
crease the  depth. 

Under  comparatively  light  traffic  a  foundation  of 
gravel  or  broken  stone  as  mentioned  in  Art.  46  may  be 
used.  This  foundation  should,  however,  usually  be 
employed  only  where  traffic  is  light  and  the  road-bed 
good. 

The  double-layer  pavement  (see  Fig.  22)  consists  of 
a  foundation  made  by  placing  a  layer  of  sand  or  gravel 
3  to  5  inches  thick  upon  the  road-bed,  rolling  it 
thoroughly  and  laying  a  course  of  bricks  upon  it.  The 
bricks  are  laid  flat  with  their  greatest  dimension  length- 
wise of  the  street,  as  explained  in  Art.  48. 

This  foundation  has  been  more  extensively  used 
under  brick  pavements  than  any  other,  and  has  often 
given  satisfactory  results.  It  is  now  largely  giving 


BRICK   PAVEMENTS.  I2Q 

place  to  concrete  in  the  better  class  of  work,  and  in 
many  cases  under  light  traffic  its  economy  is  question- 
able, as  the  layer  of  gravel  would  often  answer  equally 
well  without  the  lower  layer  of  bricks.  The  best  base 
to  use  for  a  particular  work  must  usually  be  largely 
determined  by  the  availability  of  various  materials. 

ART.  54.    CONSTRUCTION  OF  BRICK  PAVEMENT. 

In  laying  a  brick  pavement,  after  the  completion  of 
the  foundation  as  described  in  Chap.  VI  and  Art.  53, 
a  cushion  coat  of  sand  is  spread  over  the  surface  of  the 
foundation,  f  inch  to  i£  inches  thick,  to  receive  the 
surface  layer  of  bricks.  The  cushion  coat  should  be 
composed  of  clear  sharp  sand,  and  quite  dry  when  the 
surface  brick  are  placed  upon  it.  It  is  also  desirable 
that  the  sand  layer  be  rolled  with  a  light  roller  and 
brought  carefully  to  surface,  in  order  that  it  may  afford 
firm  and  even  bearing  for  the  brick. 

The  brick  surface  should  be  composed  of  carefully 
selected  material,  as  durable,  impervious,  and  uniform 
as  possible.  The  bricks  should  be  laid  on  edge,  in 
close  contact  with  each  other.  They  are  usually  arranged 
in  courses  at  right  angles  to  the  line  of  the  street,  the 
greatest  length  of  the  brick  being  across  the  street,  and 
the  bricks  in  adjoining  courses  breaking  joints  with 
each  other.  The  laying  is  begun  at  the  curb,  alternate 
courses  beginning  with  whole  and  half  bricks  and  work- 
ing to  the  centre,  the  work  at  the  curb  being  carried 
but  very  little  ahead  of  that  at  the  middle  of  the 
street,  in  order  that  partial  courses  may  not  be  dis- 
turbed before  being  completed  across  the  street.  This 
system  of  construction  is  shown  in  Fig.  21,  which  repre- 


I3O     A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 


sents  a  pavement  as  constructed  for  heavy  traffic  on 
heavy  concrete  foundation. 

In  many  cases  the  gutter-bricks  are  turned  with  the 


FIG.  21. 


greatest  dimension  lengthwise  of  the  street,  with  the 
object  of  facilitating  the  flow  of  surface-water  in  the 
gutter.  The  advantage  of  this  is  doubtful,  as  it  has 
the  effect  of  breaking  the  bond  of  the  pavement  be- 
tween the  gutter-bricks  and  roadway.  This  is  shown 
in  Fig.  22,  which  shows  the  construction  of  a  double- 


;;.;>; 


FIG.  22. 


layer  pavement  with  brick  and  gravel  base,  as  has  been 
commonly  used  under  light  or  moderate  traffic. 

Some  engineers  advocate  laying  the  brick  at  an  angle 


BRICK  PAVEMENTS.  13! 

of  45  degrees  with  the  street  line,  except  on  streets 
where  there  are  street-railway  tracks.  Mr.  Niles  Merri- 
wether,  City  Engineer  of  Memphis,  Tenn.,  in  his  report 
for  1893,  expresses  the  opinion,  based  upon  experience 
in  that  city,  that  this  arrangement  of  bricks  conduces 
to  good  wear  in  the  pavement. 

After  the  surface  layer  of  brick  is  in  position  it 
should  be  rammed  or  rolled  to  a  smooth  and  uniform 
surface.  Usually  a  heavy  wooden  rammer  is  employed 
for  this  purpose,  and  the  ramming  should  be  so 
thoroughly  done  as  to  discover  any  weak  places  that 
may  exist  in  the  pavement  by  forcing  the  bricks  down 
out  of  surface  at  such  points.  When  such  places  are 
discovered  the  bricks  should  be  removed  and  the  sand 
filled  in  below  to  properly  support  them. 

When  the  bricks  are  well  rammed  and  brought  to 
the  proper  surface  the  joints  should  be  filled  with 
material  of  an  impervious  nature  that  will  cement  the 
bricks  together  and  form  them  into  a  solid  and  imper- 
vious roadway.  Various  mixtures  of  coal-tar  and 
asphalt  are  commonly  used  for  this  purpose.  A  grout- 
ing of  hydraulic  cement  mortar  is  also  sometimes 
employed  for  this  purpose. 

After  the  joints  are  well  filled  a  coating  of  sand  J  or 
•J-  inch  thick  is  given  to  the  pavement  and  it  is  opened 
for  trafrlc.  In  some  cases  the  entire  surface  of  the 
pavement  is  coated  with  the  tar,  and  the  layer  of  sand 
is  applied  hot,  with  the  view  of  insuring  the  binding  of 
the  surface-bricks  and  rendering  the  pavement  less 
pervious. 

In  many  cases  also  the  tar  filling  is  omitted  altogether 
and  the  surface  after  ramming  is  covered  with  the  sand 
layer  and  opened  to  travel. 


132     A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

There  is  considerable  variation  in  the  methods  of 
construction  employed  in  different  localities,  as  to  the 
kind  of  foundation  used,  the  arrangement  of  the  surface- 
bricks,  and  the  filling  of  the  joints.  A  single-course 
pavement  on  a  light  gravel  foundation  with  joints  filled 
only  with  sand  or  small  gravel  has  frequently  been 
used  for  light  traffic  under  favorable  conditions  with 
satisfactory  results.  Solid  and  impervious  construction, 
however,  will  always  give  the  best  results  in  wear,  and 
usually  will  be  most  economical  in  the  end. 

ART.  55.    MAINTENANCE  OF  BRICK  PAVEMENTS. 

The  maintenance  necessary  for  a  brick  pavement 
consists  in  keeping  it  clean  and  carefully  watching  it, 
especially  during  the  first  year  or  two  years,  to  see  that 
no  breaks  occur  due  to  the  use  of  defective  bricks  in 
the  surface  or  to  insufficient  support  from  the  founda- 
tion at  any  point.  When  any  unevenness  from  either 
of  these  causes  appears,  it  should  be  at  once  rectified 
before  the  pavement  becomes  irregularly  worn  in  con- 
sequence. 

While,  as  already  stated,  the  utmost  care  should 
always  be  taken  to  use  only  material  of  a  uniform 
quality  in  the  surface  of  the  pavement,  still  under  the 
closest  inspection  some  inferior  material  may  be  used, 
which  will  only  be  shown  when  wear  comes  on  the 
pavement,  and  unless  then  removed  at  once  it  will 
cause  the  evenness  of  the  surface  to  be  impaired  about 
it.  Irregular  support  from  the  foundation  will  be 
less  likely  to  occur  in  good  construction,  but  its  effect 
will  be  similar  to  defective  material,  the  sinking  of  in- 
dividual bricks  producing  uneven  wear.  Weak  spots 


BRICK   PAVEMENTS.  133 

in  the  foundation  may  sometimes  be  caused,  where  con- 
crete foundation  is  not  employed,  by  surface-water 
which  is  permitted  to  pass  through  the  joints,  saturat- 
ing the  sand  or  gravel  beneath  and  causing  it  to  move 
under  concentrated  loads.  For  this  reason  the  joints 
should  be  observed  during  the  early  wear  of  the  pave- 
ment in  order  to  remedy  any  case  where  they  may  not 
have  been  properly  filled. 

Where  a  brick  pavement  has  been  constructed  of 
good  material  and  kept  in  good  surface  during  the 
early  period  of  use,  it  may  then  reasonably  be  expected 
to  wear  out  without  any  considerable  expense  for  small 
repairs.  The  length  of  time  the  pavement  may  be  ex- 
pected to  wear  depends  upon  the  quality  of  the  ma- 
terial and  the  methods  of  construction.  For  the 
moderate  traffic  of  many  of  the  smaller  cities,  and 
lesser  streets  in  the  large  cities,  brick  has  shown  an 
endurance  which  indicates  it  to  be  a  satisfactory  and 
economical  material,  and  it  is  not  improbable  that  by 
careful  attention  to  proper  construction  it  may  be  used 
for  even  heavier  traffic. 


CHAPTER  VIII. 
ASPHALT  PAVEMENTS. 

ART.  56.    ASPHALTUM. 

ASPHALTUM  is  a  mineral  pitch  which  occurs  in 
a  number  of  localities  widely  distributed  over  the 
surface  of  the  earth.  It  is  supposed  by  most  authori- 
ties to  be  the  result  of  the  decomposition  of  vegetable 
matter,  although  by  some  it  is  considered  to  be  of  vol- 
canic origin.  It  consists,  in  its  natural  state,  of  bitumen 
with  a  small  percentage  of  foreign  organic  matter  and 
mixed  with  more  or  less  mineral  earth,  and  varies 
according  to  the  nature  of  the  bitumen  from  the  soft 
viscid  condition  of  mineral  tar  to  a  hard  brittle  sub- 
stance of  glassy  appearance  and  conchoidal  fracture. 
It  also  occurs  in  a  solid  state  as  a  rock  impregnated 
with  bitumen,  which  will  be  separately  considered  in 
another  article  under  the  head  of  rock  asphalt. 

The  asphaltum  used  for  street  pavements  in  this 
country  is  obtained  for  the  most  part  from  the  island 
of  Trinidad,  W.  I.,  and  from  the  state  of  Bermudez, 
Venezuela.  This  asphaltum  is  known  as  lake  asphalt 
or  as  land  asphalt  according  to  the  source  from  which 
it  is  derived.  Lake  aspJialt  is  found  in  large  deposits 
known  as  the  pitch  lakes.  The  lakes  cover  a  consider- 
able areaj — in  Trinidad  about  100  acres  ;  in  Bermudez 

134 


ASPHALT   PAVEMENTS.  135 

several  hundred, — and  the  pitch  seems  to  be,  or  have 
been,  forced  upward  from  below  through  fissures  in  the 
rock  or  craters.  The  pitch  upon  contact  with  the  air 
gives  off  gas  and  gradually  hardens.  In  the  lakes 
proper  the  asphaltum  is  more  or  less  in  motion,  and 
excavations  in  the  surface  are  soon  filled  by  the  flow 
of  material  from  the  sides  and  bottom.  The  pitch  near 
the  centre  of  the  lake  at  Trinidad  is  more  soft  than 
near  the  sides,  and  it  has  'been  supposed  that  the 
supply  from  subterranean  sources  still  continues  to 
some  extent.  It  has  also  been  found  that  the  surface 
of  the  lake  is  higher  in  the  centre  than  at  the  sides, 
and  that  the  general  elevation  of  the  surface  has  been 
lowered  somewhat  by  the  large  quantities  of  material 
which  have  been  removed  from  it. 

The  so-called  land  asphalt  from  Trinidad  is  found  in 
vicinity  of  the  lake,  and  is  a  harder  material  than  the 
lake  asphalt,  probably  from  longer  exposure  to  the  air. 
It  may  have  been  derived  either  from  overflow  of  the 
lake  or  from  independent  subterranean  sources  the 
action  in  which  has  now  ceased.  (For  a  complete 
description  of  the  Trinidad  pitch  deposits  see  the 
"  Report  of  the  Inspector  of  Asphalts  and  Cements  of 
the  District  of  Columbia  "  for  1891-92.) 

Asphaltum  similar  in  character  to  that  already  men- 
tioned occurs  at  many  other  places.  Mines  are  worked 
in  Cuba  and  Peru,  and  large  deposits  are  found  in 
Mexico.  It  occurs  at  a  number  of  places  in  Europe, 
while  the  bituminous  mortar  of  the  ancient  Chaldean 
constructions  was  of  this  character,  and  beds  of  as- 
phaltum  are  still  found  in  that  country  from  which  it 
may  have  been  obtained.  In  the  United  States  de- 
posit? of  asphaltum  are  found  in  California,  Utah, 


136     A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

West  Virginia,  and  other  places.  Those  of  California 
have  to  some  extent  been  applied  to  paving  purposes, 
but  have  not  as  yet  been  largely  employed. 

The  crude  asphaltum  usually  contains  considerable 
water  as  well  as  earthy  and  vegetable  impurities.  It  is 
heated  in  a  boiler  to  300°  or  400°  Fahr.,  the  water 
being  driven  off,  and  the  impurities  settling  to  the 
bottom  or  forming  a  scum  on  top.  The  liquid  as- 
phaltum is  then  drawn  off  and  is  known  as  refined 
asphalt.  This  refined  asphalt  may  contain  more  or 
less  mineral  or  earthy  matter  distributed  through  it  in 
a  finely  divided  state.  Refined  Trinidad  asphalt  may 
contain  52  to  56  per  cent  of  pure  bitumen,  while  Ber- 
mudez  asphalt  is  said  to  contain  97  per  cent. 

The  refined  asphalt  is  brittle  at  ordinary  tempera- 
tures, and  possesses  little  cementitious  value.  To  re- 
duce it  to  a  condition  from  which  it  may  be  easily 
compacted  in  the  pavement,  it  is  heated  to  a  tempera- 
ture of  about  300°  Fahr.  and  mixed  with  the  oil  re- 
siduum obtained  from  the  distillation  of  petroleum. 
This  mixture  is  very  thoroughly  worked,  so  as  to  form 
a  material  of  uniform  consistency.  The  product  is  then 
known  as  asphaltic  paving  cement. 

Great  care  is  necessary  in  mixing  the  paving  cement 
to  properly  proportion  the  ingredients,  as  the  value  of 
the  cement  depends  upon  their  nice  adjustment.  Both 
materials  are  quite  variable  in  their  properties,  and 
continual  tests  are  necessary  of  the  materials  employed 
as  well  as  of  the  resulting  product,  in  order  to  obtain 
a  cement  of  uniformly  good  quality.  The  quantity  of 
oil  necessary  varies  with  the  nature  of  the  asphaltum, 
being  less  as  the  asphaltum  is  of  a  more  plastic  and 
less  refractory  nature. 


ASPHALT   PAVEMENTS.  137 

The  surface  material  for  a  pavement  of  asphaltnm  is 
formed  by  mixing  asphaltic  paving  cement,  prepared 
as  already  described,  with  sand,  or  with  sand  and  pow- 
dered limestone.  The  proportions  of  the  various  in- 
gredients depend  upon  their  character  and  upon  the 
requirements  of  the  pavement.  In  order  to  obtain 
good  results  it  is  necessary  that  the  exact  nature  of 
each  of  the  ingredients  be  determined,  and  the  proper 
amounts  used.  With  Trinidad  asphaltum  the  surface 
material  ordinarily  contains  about  10  per  cent  of  pure 
bitumen. 

The  sand  used  for  this  purpose  is  usually  very  fine, 
60  per  cent  being  sometimes  required  to  pass  a  sieve 
of  60  meshes  per  linear  inch,  and  all  of  it  through  one 
of  30  meshes.  It  is  important  also  that  the  sand  be 
clean  and  free  from  loam  or  clay.  The  carbonate  of 
lime  is  used  in  the  form  of  fine  powder. 

In  forming  the  surface  material  the  sand  and  paving 
cement  are  separately  heated  to  a  temperature  of 
about  300°  Fahr.  and  mixed,  while  hot,  in  an  apparatus 
which  thoroughly  incorporates  them  into  the  mixture. 
When  powdered  limestone  is  used  it  is  added  cold  to 
the  hot  sand  before  mixing  with  the  hot  paving 
cement. 

As  may  be  readily  seen,  the  selection  of  asphaltum 
for  paving  purposes,  as  well  as  the  process  of  forming 
the  surface  material,  is  a  matter  requiring  very  great 
care  and  an  intimate  knowledge  of  the  characteristics 
of  the  materials  to  be  employed.  In  order  to  secure 
good  results  it  is  quite  essential  that  careful  examina- 
tion be  made  of  every  mixing  of  the  material  for  the 
surface,  both  by  testing  the  materials  before  they  are 
used  and  the  product  after  it  is  formed.  Analysis  of 


138     A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

every  lot  should  be  made  as  well  as  consistency  tests. 
Experience  is  the  only  guide,  and  there  is  no  method 
of  judging  of  value  in  such  work  other  than  by  the  re- 
sults of  former  work  of  the  same  kind. 

Variations  in  the  quality  of  asphaltum  are  attributed 
to  the  character  as  well  as  the  quantity  of  bitumen 
that  it  contains.  This  bitumen  is  made  up  of 
two  parts,  the  first  of  which,  called  petrolene,  is  the 
oily  and  cementitious  material ;  the  other,  called 
asphaltene,  is  the  hard  material  lacking,  in  the  ce- 
menting properties.  The  varying  proportions  of  these 
ingredients  in  the  bitumen  determine  the  temperature 
at  which  it  will  melt,  and  the  facility  with  which  it  may 
be  used  for  paving  purposes.  A  certain  proportion  of 
asphaltene  is  probably  necessary  to  give  the  material 
sufficient  stiffness  at  ordinary  temperatures;  but  too 
large  a  quantity  makes  it  lacking  in  plasticity  and 
cementing  properties,  and  renders  it  necessary  to  use 
a  large  proportion  of  residuum  oil  in  preparing  the 
asphaltic  cement. 

The  residuum  oil  also  varies  considerably  iivcharacter, 
and  the  quantity  to  be  used  in  each  case  depends 
somewhat  upon  the  character  of  the  oil.  The  object 
aimed  at  is  to  get  a  cement  of  given  consistency,  which 
is  measured  by  the  penetration  of  a  needle,  at  a  stand- 
ard temperature  and  under  a  standard  weight.  In 
forming  the  paving  cement  it  is  necessary  that  the 
materials  be  constantly  agitated  in  order  that  the  oil 
and  asphaltum  may  be  intimately  mixed,  and  that  the 
mass  of  cement  may  be  uniform  throughout. 

For  making  a  paving  cement,  in  some  cases  where 
American  asphalt  has  been  used,  a  maltha,  or  liquid 
bitumen,  has  been  substituted  for  the  residuum  oil  to 


ASPHALT   PAVEMENTS.  139 

reduce  the  asphaltum  to  plasticity.  This  maltha  is  a 
nearly  pure  bitumen,  similar  in  character  to  the  asphal- 
tum, but  differing  in  that  the  bitumen  contains  a  high 
percentage  of  the  hydrocarbon  known  as  petrolene 
and  but  very  little  asphaltene,  the  proportion  between 
the  two  being  such  as  make  the  bitumen  liquid  at 
ordinary  temperatures. 

The  composition  of  surface  material  for  asphalt 
pavements  must  be  varied  to  suit  the  conditions  under 
which  each  is  built.  The  variations  of  temperature 
to  which  the  pavement  may  be  subjected  are  of  special 
importance,  and  the  nature  of  the  traffic  may  also  have 
an  influence.  The  surface  must  not  soften  under  the 
heat  of  summer,  and  yet  must  be  sufficiently  plastic 
not  to  become  brittle  and  chip  off  in  cold  weather. 
For  light  traffic  the  material  may  be  more  soft  in  warm 
weather  than  under  heavy  traffic,  as  it  is  not  so  liable 
to  cutting  under  the  loads. 

The  quantity  of  paving  cement  used  in  the  surface 
material  may  depend  somewhat  upon  the  consistency 
adopted  for  the  cement.  If  it  be  stiff  a  larger  per- 
centage may  be  used,  with  the  same  effect  as  to  the 
softening  of  the  surface  in  summer  than  if  it  be  soft. 
The  common  proportions  of  ingredients  employed  in 
forming  the  surface  material  are  approximately  as  fol- 
lows : 

For  Trinidad  Asphalt.  For  Bermudez  Asphalt. 

Asphaltic  cement     12  to  15  per  cent.     9  to  10  per  cent 

Sand 83  to  70        "          71  to  60 

Carbonate  of  lime       5  to  15         "  20  to  30        " 

In  some  cases  stone-dust,  formed  by  crushing  a  hard 
stone,  such  as  granite,  to  about  the  dimensions  of  a 


I4O     A  TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

fine  sand,  is  substituted  for  a  portion  of  the  sand  and 
carbonate  of  lime. 

The  proper  treatment  in  any  instance  can  only  be 
determined  by  a  careful  study  of  the  materials  to  be 
used,  the  climatic  conditions,  and  the  service  required. 

ART.  57.    ROCK  ASPHALT. 

Rock  asphalt,  as  commonly  used  in  paving,  consists 
of  limestone  naturally  impregnated  with  bitumen  in 
such  proportion  as  to  form  a  material  which  may  be 
softened  by  the  action  of  heat  and  again  consolidate 
when  cooled  if  brought  under  pressure. 

This  rock  is  mined  at  several  places  in  Europe, 
notably  at  Seyssel,  France ;  Travers,  Switzerland  ; 
Ragusa,  Sicily,  and  Vorwohle,  Germany.  It  is  usually 
composed  of  nearly  pure  carbonate  of  lime,  impreg- 
nated with  from  7  to  20  per  cent  of  bitumen.  It 
occurs  in  veins,  after  the  manner  of  coal,  is  hard  at  the 
ordinary  temperatures  of  the  mines,  and  is  quarried  by 
the  use  of  explosives. 

The  preparation  of  the  surface  material  of  rock 
asphalt  consists  only  in  crushing  and  grinding  the 
rock  to  powder  and  heating  the  powder  to  drive  off  the 
water  and  soften  it,  so  that  it  may  be  compacted  in 
the  roadway.  The  powder  is  heated  to  a  temperature 
of  from  200°  to  300°  Fahr.,  and  is  applied  while  hot  in 
laying  the  surface. 

Natural  rock  asphalt,  suitable  for  paving  purposes, 
usually  contains  from  9  to  12  per  cent  of  bitumen.  If 
it  contain  much  more  than  this  it  is  apt  to  become 
soft  in  warm  weather.  If  it  contain  less  it  will  not 
consolidate  properly  or  bind  well  in  the  pavement. 


ASPHALT   PAVEMENTS.  14! 

The  rock  should  be  of  fine  even  grain,  and  have  the 
bitumen  uniformly  distributed  through  it.  In  form- 
ing the  surface  material  for  rock  asphalt  pavements, 
the  rock  from  different  mines  is  commonly  mixed  in 
such  proportions  as  to  give  about  10  or  12  per  cent  of 
bitumen  to  the  mixture,  thus  making  a  harder  surface 
than  would  be  obtained  by  the  use  of  the  rich  rock 
alone,  as  well  as  less  likelihood  of  softening.  No  other 
material  is  mixed  with  the  rock  in  forming  the 
surface. 

In  determining  a  mixture  of  asphalt  rock,  as  in  the  use 
of  the  lake  asphalt,  the  local  conditions  of  climate  and 
traffic  must  be  considered,  and  the  quantity  of  bitumen 
be  so  proportioned  as  to  remain  solid  in  summer  and 
not  become  brittle  and  lose  cohesion  in  winter.  Ex- 
perience with  the  material  and  exercise  of  great  care 
in  the  determination  of  proper  proportions  is  therefore 
essential  to  success  in  the  construction  of  any  asphalt 
pavement. 

In  the  use  of  bituminous  limestone  for  sidewalks 
and  many  other  purposes  where  a  plastic  material  is 
required,  the  rock  asphalt  powder  is  mixed  with  an 
additional  quantity  of  bitumen,  or  asphaltum,  sufficient 
commonly  to  give  a  product  containing  15  to  1 8  per 
cent  of  bitumen.  This  product  is  known  as  asphalt 
mastic  in  Europe.  For  use  in  sidewalks  the  mastic  is 
melted  and  mixed  with  sand  or  gravel  to  form  a  wear- 
ing surface. 

A  sandstone  impregnated  with  bitumen  occurs  at  a 
number  of  localities  in  the  United  States.  This  stone 
has  been  applied  to  some  extent  in  paving,  in  some- 
what the  same  manner  as  the  European  material.  It 


142     A  TEXT-BOOK  ON  ROADS  AND   PAVEMENTS. 

is  still  in  the  experimental  stage,  and  has  not  come  into 
general  use. 

ART.  58.    ASPHALT  BLOCKS. 

Asphalt  paving  blocks  are  commonly  formed  of  a 
mixture  of  asphaltic  cement  with  crushed  limestone. 
This  limestone  is  crushed  to  sizes  of  £  inch  in  diameter 
or  less,  and  mixed  with  the  asphaltic  paving  cement 
formed  as  described  in  Art.  56,  in  such  proportions 
as  that  the  product  contains  about  10  per  cent  of 
bitumen. 

The  materials  are  heated  to  a  temperature  of  about 
300°  Fahr.,  and  mixed  while  hot  in  an  apparatus 
arranged  to  secure  the  even  distribution  of  the  ingre- 
dients through  the  mass.  The  thorough  incorporation 
of  the  various  materials  in  the  mixture  is  of  first  im- 
portance in  producing  homogeneous  and  uniform 
blocks,  while  the  quality  of  the  materials  used  needs 
as  careful  inspection  as  in  the  case  of  the  surface  mate- 
rial for  sheet  pavements. 

When  the  mixing  is  complete,  the  material  is  placed 
in  moulds  and  subjected  to  heavy  pressure,  after  which 
the  blocks  are  cooled  suddenly  by  plunging  into  cold 
water. 

These  blocks  have  usually  been  made  larger  than 
paving-bricks,  the  common  size  being  12  inches  long, 
4  inches  wide,  and  5  inches  deep.  They  are  laid  in  the 
same  manner  as  brick,  as  closely  in  contact  as  possible, 
and  driven  together.  Under  the  action  of  the  sun  and 
the  traffic,  the  asphalt  blocks  soon  become  cemented 
together,  through  the  medium  of  the  asphaltic  cement, 
and  form,  like  the  sheet  asphalt  pavements,  a  practi- 
cally imperivous  surface. 


ASPHALT   PAVEMENTS.  143 

They  are  commonly  used  on  streets  of  light  traffic 
only,  as  the  blocks  as  heretofore  constituted  wear 
rapidly  under  heavy  traffic.  They  are  usually  laid 
upon  a  foundation  of  sand  or  of  sand  and  gravel,  and 
on  account  of  the  impervious  nature  of  the  surface 
may  often  give  satisfactory  results  on  such  a  founda- 
tion, where  a  more  pervious  block  pavement  or  a  sheet 
pavement  would  require  more  efficient  support. 

A  large  amount  of  pavement,  of  blocks  made  prac- 
tically as  described  above,  has  been  laid  in  this  country, 
particularly  at  Baltimore  and  Washington,  and  have 
shown  good  durability  in  wear  under  moderate  and 
light  travel. 

These  blocks  have  the  advantage  over  sheet  asphalt 
for  the  smaller  cities,  that  the  blocks  may  be  formed 
at  a  central-point  and  shipped  ready  for  use  to  the  site 
of  the  proposed  pavement,  and  that  no  special  plant 
need  be  erected  in  each  town  where  they  are  to  be 
constructed. 

In  forming  the  asphalt  block  pavement  the  road-bed 
is  brought  to  subgrade  in  the  ordinary  manner  and 
rolled,  leaving  room  for  the  pavement  of  uniform  thick- 
ness to  be  placed  upon  it.  A  layer  of  gravel  4  or  5 
inches  deep  is  then  placed  and  rolled,  with  a  cushion 
coat  of  sand  I  to  2  inches,  and  then  the  paving  blocks. 
The  blocks  are  pressed  together  in  the  courses  by  the 
use  of  a  lever,  and  the  courses  driven  against  each 
other  with  a  maul  to  reduce  the  joints  as  much  as 
possible.  A  coating  of  sand  is  given  to  the  surface  of 
the  pavement,  and  it  is  rammed  to  a  firm  and  uniform 
surface,  as  in  the  case  of  brick. 


144    A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

ART.  59.    FOUNDATIONS  FOR  SHEET  PAVEMENTS. 

As  a  sheet  asphalt  surface  has  no  power  to  sustain 
loads,  acting  only  as  a  wearing  surface,  which  must  be 
held  in  place  from  below,  it  is  essential  that  it  be 
placed  upon  a  very  firm,  unyielding  foundation.  It  is 
consequently  nearly  always  placed  upon  a  concrete 
base,  which  is  commonly  formed  of  hydraulic  cement 
mortar  and  broken  stone,  prepared  as  described  in 
Art.  47.  In  the  use  of  this  base,  it  is  necessary  that 
the  mortar  be  fully  set,  and  the  concrete  thoroughly 
dry  before  the  asphalt  is  laid  upon  it,  as  the  placing  of 
the  hot  surface  material  upon  a  damp  foundation  will 
cause  the  blistering  and  possible  disintegration  of  the 
surface  by  the  steam  generated  from  the  base  by  the 
heat  of  the  material. 

For  moderate  or  heavy  traffic  in  cities,  the  concrete 
base  is  commonly  made  6  inches  thick.  For  lighter 
traffic  a  less  depth,  4  inches  or  5  inches,  is  sometimes 
employed.  The  depth  necessary  will  depend  upon  the 
nature  of  the  road-bed  as  well  as  the  weight  of  the 
traffic.  It  should  be  greater  as  the  subsoil  is  less  firm 
and  well  drained. 

Frequently  the  concrete  for  the  foundation  is  formed 
of  asphaltic  or  coal  tar  paving  cement  instead  of 
hydraulic  cement  mortar.  It  is  then  known  as  bitu- 
minous concrete,  and  the  foundation  is  called  a  bitumi- 
nous base  to  distinguish  it  from  the  ordinary  hydraulic 
base.  The  advantage  claimed  for  the  bituminous  base 
is  that  the  foundation  and  surface  material  become 
joined  into  a  single  mass,  with  the  effect  of  anchoring 
the  surface  and  preventing  the  formation  of  weather- 
cracks  and  wave-surfaces,  which  sometimes  occur  when 


ASPHALT  PAVEMENTS.  145 

the  hydraulic  base  is  used,  under  a  light  surface  layer, 
in  consequence  of  the  lack  of  bond  between  the  hy- 
draulic concrete  and  the  asphalt  surface.  Another 
reason  for  the  use  of  this  base  is  that  it  is  cheaper  than 
the  hydraulic  base. 

An  intermediate  layer  known  as  the  binder  course 
is  now  commonly  added  to  the  foundation,  or  rather 
placed  between  the  foundation  and  surface  layers. 
This  layer  is  usually  formed  of  coal-tar  or  asphaltic 
paving  cement,  mixed  with  small  broken  stone,  not 
more  than  I  inch  in  diameter,  about  one  gallon  of  the 
cement  being  required  to  I  cubic  foot  of  stone. 

The  materials  are  mixed  hot  and  laid  and  rolled  in 
the  same  manner  as  the  surface  layer.  This  binder 
becomes  consolidated  with,  and  gives  added  depth  and 
strength  to,  the  surface  ;  thus  having  a  tendency  to  pre- 
vent the  cracks  and  wave-surfaces  which  may  other- 
wise appear  in  the  surface  when  used  upon  an  hydraulic 
base. 

The  hydraulic  base  is  usually  preferred  to  the  bi- 
tuminous base  on  account  of  its  forming  an  unyield- 
ing structure,  not  likely  to  be  forced  out  of  place  by 
the  weight  of  the  traffic  at  any  point  where  the  support 
of  the  road-bed  may  be  weakened,  while  the  bituminous 
concrete  has  not  the  strength  to  resist  deformation  un- 
der heavy  loads  unless  uniformly  supported.  The  join- 
ing of  the  base  and  surface  into  one  mass,  as  is  effected 
by  the  bituminous  base  is  also  a  disadvantage  when  the 
pavement  is  to  be  resurfaced,  as  with  the  hydraulic 
concrete  base  the  surface  may  be  easily  stripped  off, 
and  a  new  surface  placed  without  injury  to  the  founda- 
tion. 


. 

4 


146    A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  60.    CONSTRUCTION  OF  SHEET  PAVEMENTS. 

The  construction  of  asphalt  pavements  in  this 
country  is,  in  the  main,  in  the  hands  of  two  or  three 
large  corporations,  and  methods  of  construction  vary 
but  little,  the  differences  in  the  various  pavements  being 
principally  due  to  differences  in  the  composition  of  the 
materials  used. 

In  constructing  the  pavement  after  the  completion 
of  the  foundation  as  indicated  in  the  preceding  article, 
the  surface  material  is  brought  to  the  place  where  it  is 
to  be  used  in  a  large  kettle  and  applied  in  a  hot,  semi- 
plastic  condition  and  thoroughly  consolidated  by  roll- 
ing. The  tools  with  which  the  material  is  handled  are 
kept  hot,  hot  rakes  being  ordinarily  employed  for 
spreading  it,  and  hot  rollers  for  the  first  compacting. 

Sometimes,  as  already  indicated,  a  binder  course  is 
inserted  between  the  base  and  surface  layers.  This 
binder  course  is  usually  ij  inches  thick,  composed  of 
coal-tar  cement  and  small  broken  stone,  and  applied  in 
the  same  manner  as  the  surface  layer.  This  construc- 
tion has  been  considerably  used  in  Washington,  and  a 
pavement  so  constructed  is  known  as  the  combination 
pavement. 

In  some  cases  where  the  binder  course  is  omitted, 
the  surface  is  applied  in  two  layers,  of  which  the  lower, 
known  as  the  cushion  coat,  is  made  from  £  inch  to 
I  inch  thick  when  compacted,  and  contains  a  higher 
percentage  of  paving  cement  than  the  surface  layer. 
This  layer  having  more  of  the  cementitious  material 
adheres  more  strongly  to  the  hydraulic  base,  and  forms 
a  tie  between  foundation  and  surface. 

When  a  bituminous  base  is  employed  the  cushion 


ASPHALT  PAVEMENTS.  147 

coat  is  generally  considered  unnecessary,  as  the  surface 
layer  will  join  directly  with  the  base.  The  binder 
course  is,  however,  frequently  used  in  this  case  to  give 
added  strength  and  weight  to  the  pavement. 

The  surface  coat,  prepared  as  described  in  Arts.  56 
and  57,  is  usually  applied  so  as  to  be  about  I \  to  2^ 
inches  thick  when  compacted  in  the  pavement.  When 
a  binder  course  is  employed  about  I  j-  inches,  otherwise 
2  inches  to  2j  inches. 

In  the  construction  of  rock  asphalt  pavements,  which 
have  been  almost  exclusively  used  in  Europe,  it  is  com- 
mon to  ram  the  surface  layer  with  hot  rammers,  and 
smooth  it  off  with  smoothing-irons,  while  in  this  country 
small  hot  rollers  have  usually  been  employed.  The 
practice  of  the  rock  asphalt  companies  has  also  differed 
from  that  of  those  using  lake  asphalt,  in  that  the  latter 
continue  rolling  the  surface  with  heavy  rollers  until  it 
is  hardened  and  shows  no  mark,  while  the  European 
practice  is  to  roll  more  lightly  and  leave  the  final  com- 
pression to  be  given  by  the  traffic. 

In  Europe  foundations  of  hydraulic  cement  concrete 
are  exclusively  used,  with  the  surface  layer  usually 
directly  in  contact  with  it.  This  system  is  also  most 
commonly  used  in  this  contry,  the  standard  pavement 
being  formed  of  an  hydraulic  base  6  inches  thick  with 
a  single  surface  layer  2\  inches  thick  when  compacted, 
or  where  lighter  construction  is  admissible,  an  hydrau- 
lic base  4  inches  thick  is  used  with  a  surface  layer  2 
inches  in  thickness. 

In  all  asphalt  pavements  it  is  customary  during  the 
rolling  to  give  the  surface  a  coating  of  hydraulic  cement, 
which  is  usually  swept  lightly  over  the  surface. 


148    A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  61.    VULCANITE  OR  DISTILLATE  PAVEMENT. 

Numerous  attempts  have  been  made  to  construct  a 
pavement  by  the  use  of  coal-tar  as  a  cementing  material 
.in  place  of  or  in  conjunction  with  asphaltum.  But  few 
of  these  have  met  with  any  degree  of  success.  Coal- 
tar  by  itself,  as  most  commonly  employed,  is  soon  dis- 
integrated by  the  action  of  the  weather ;  it  is  also 
strongly  affected  by  temperature,  becoming  soft  in  hot 
weather  and  brittle  in  cold  weather. 

Pavements  in  which  the  wearing  surface  is  composed 
of  a  mixture  of  coal-tar  and  asphaltum  have  in  some 
cases  given  good  results  in  practice.  These  pavements 
are  known  as  vulcanite  or  coal-tar  distillate  pavements. 
They  are  much  cheaper  than  asphalt.  They  are  said 
to  be  somewhat  less  slippery,  and  to  resist  better  where 
exposed  to  dampness.  The  vulcanite  surface,  however, 
is  not  so  durable  under  wear,  and  it  requires  very  great 
care  in  construction  to  produce  a  surface  of  uniformly 
good  quality,  because  of  the  possible  variations  in  the 
nature  of  the  coal-tar  used. 

In  the  preparation  of  the  surface  material  for  distil- 
late pavement  as  it  has  been  used  in  Washington,  a 
paving  cement  is  used  containing  70  to  75  per  cent  of 
coal-tar  paving  cement  and  25  to  30  parts  refined  as- 
phaltum. The  construction  of  the  pavement  is  in  other 
respects  similar  to  that  of  the  combination  asphalt 
pavement  on  a  bituminous  base.  As  commonly  con- 
structed the  pavement  includes  a  4-inch  bituminous 
base,  a  i£-inch  binder-course,  and  a  surface  of  tar  dis- 
tillate ij-  inches  thick.  The  material  for  the  wearing 
surface  for  this  pavement,  according  to  the  specifica- 
tions of  1892,  was  formed  as  follows: 


ASPHALT   PAVEMENTS.  149 

Clean  sharp  sand 63  to  58  per  cent. 

Broken  stone  or  rock  dust.  .23  to  28    "       " 

Paving  cement 13  to  15    "       " 

Hydraulic  cement 0.9         "       " 

Slaked  lime 0.15         "       " 

Flour  of  sulphur o.i         "       " 

The  materials  are    heated   to   250°  Fahr.  and    mixed 
hot,  then  laid  after  the  manner  of  asphalt. 

ART.  62.    MAINTENANCE  OF  ASPHALT  PAVEMENTS. 

To  give  good  service  asphalt  pavements  must  be 
kept  clean.  On  account  of  the  smooth  surface  and 
absence  of  joints,  cleaning  may  be  readily  accomplished  ; 
and  the  presence  of  dirt,  especially  in  wet  weather 
when  it  is  likely  to  cause  the  surface  to  remain  damp,  is 
liable  to  cause  the  asphalt  to  rot.  More  than  any 
other  pavement,  therefore,  the  durability  and  wear  of 
an  asphalt  surface  depends  upon  its  cleanliness.  The 
presence  of  dirt  upon  asphalt  in  damp  weather  is  also 
important  in  its  effect  upon  the  slipperiness  of  the 
pavement. 

Small  repairs  of  any  breaks  that  may  occur  in  an 
asphalt  surface  may  be  easily  made,  and  such  repairs 
should  be  constantly  attended  to  in  order  to  keep  the 
surface  in  good  condition.  Small  breaks  will  rapidly 
extend  if  they  are  not  repaired  at  once.  In  making 
repairs  to  the  surface  of  the  pavement  it  is  necessary  to 
cut  away  the  surface  for  a  short  distance  about  the  im- 
perfect spot,  stripping  the  surface  from  the  foundation 
and  cutting  the  layer  down  square  at  the  edges,  after 
which  a  new  piece  of  surface  may  be  introduced  to  fill 
the  hole  in  the  same  manner  that  the  original  surface 


I5O     A   TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

was  constructed.  Such  a  patch  may  ordinarily  be  put 
on  so  as  to  make  joints  that  will  join  perfectly  with  the 
old  pavement  and  not  show  where  it  has  been  placed. 
When  a  surface  has  become  so  worn  that  patches  would 
be  numerous,  the  old  surface  may  be  stripped  off  and 
a  new  one  placed  upon  the  original  foundation.  When 
repairs  are  to  be  made  upon  a  pavement  having  a  bitu- 
minous base  it  is  more  difficult  to  cut  out  the  holes  in 
satisfactory  shape,  as  there  is  no  well-defined  joint  be- 
tween the  base  and  the  surface  layers. 

The  repairs  that  may  be  required  upon  an  asphalt 
pavement  depend,  of  course,  upon  the  solidity  of  con- 
struction and  the  nature  of  the  surface  material.  There 
is  so  great  variation  in  the  materials  employed  for  the 
wearing  surface  that,  as  would  naturally  be  expected, 
very  considerable  difference  in  wear  is  shown  by  dif- 
ferent pavements.  Asphalt  pavements  for  the  most 
part,  as  has  been  stated,  are  built  by  corporations  em- 
ploying corps  of  expert  workmen,  and  the  plan  usually 
adopted  is  to  require  the  contractor  to  keep  the  pave- 
ment in  repair  for  a  certain  length  of  time  without 
charge  and  afterwards  to  maintain  it  for  a  certain  longer 
period  at  a  fixed  annual  price.  This  makes  it  an  object 
for  the  contractor  to  do  good  work,  and  is  the  mos 
effective  way  of  securing  it  where  so  many  elements  of 
uncertainty  enter. 


CHAPTER 
WOOD   PAVEMENTS. 

ART.  63.    WOOD  BLOCKS. 

WOOD  for  pavements  should  be  close-grained  and 
not  too  hard.  It  should  be  as  homogeneous  as  pos- 
sible in  order  that  the  wear  may  be  uniform,  and  soft 
enough  that  it  may  not  wear  smooth  and  slippery.  To 
give  good  service  in  wear  the  wood  should  be  pene- 
trated by  water  as  little  as  possible  and  show  good 
resistance  to  decay  under  the  action  of  the  weather. 

Wood  for  this  use  should  be  sound  and  well  seasoned. 
The  blocks  should  always  be  subjected  to  careful  in- 
spection. All  sapwood  needs  to  be  removed  in  order 
to  lessen  the  liability  to  early  decay,  and  blocks  con- 
taining shakes  and  knots  should  be  rejected. 

In  the  United  States  cedar  has  been  most  largely 
used  for  this  purpose  and  has  proved  to  be  a  quite 
satisfactory  wood  for  such  use.  Yellow  pine  and  tam- 
arack have  also  been  employed  for  pavements  to  some 
extent  at  the  North,  and  cypress,  juniper,  cottonwood, 
and  mesquite  at  the  South.  These  varieties  have  all 
been  used  with  some  success,  and  can  be  made  to  fur- 
nish a  fairly  good  paving  material. 

Oak  and  other  hard  woods  are  less  likely  to  wear 
evenly  in  the  pavement,  become  smooth  ancl  slippery, 
and  bear  less  well  the  exposure  to  the  weather  and  in- 


152     A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

fluences  tending  to  cause  decay  in  the  wood.  In  Wash- 
ington, D.  C.,  a  pavement  of  hemlock  blocks  was  at  one 
time  constructed  on  quite  an  extensive  scale,  but  proved 
unsatisfactory  and  was  soon  destroyed.  It  does  not 
appear,  however,  to  have  been  a  well-constructed  pave- 
ment or  of  properly  selected  material. 

In  Australia  hard-wood  blocks  have  been  quite  ex- 
tensively used  and  are  reported  as  giving  good  service, 
although  they  are  admitted  to  be  somewhat  slippery  in 
wet  weather.  Australian  Karri  and  Jarrah  woods  are 
employed,  and  it  is  claimed  for  them  that  they  show 
unusually  great  resistance  to  wear  and  are  not  soon 
affected  by  decay. 

In  London,  where  wood  pavements  have  been  very 
extensively  employed,  Swedish  yellow  deal  is  com- 
monly placed  at  the  head  of  the  list  of  woods  in  value, 
yellow  pine  and  Baltic  fir  being  also  largely  used  and 
considered  good  in  use.  The  Australian  woods  above 
mentioned  have  also  been  used  to  some  extent  in  Lon- 
don, and  are  said  to  have  given  very  satisfactory  ser- 
vice, showing  greater  resistance  to  wear  than  deal  or 
pine,  although  somewhat  expensive. 

Wood  pavements  are  commonly  constructed  of  blocks 
set  with  the  fibres  vertical,  so  that  wear  comes  upon  the 
ends  of  the  fibres  and  has  no  tendency  to  split  pieces 
off  from  the  blocks.  Cedar  blocks  are  used  in  the  form 
of  whole  sections  of  the  tree,  on  account  of  the  liability 
of  the  wood  to  split  off  between  the  layers  when  cut  to 
a  rectangular  shape.  They  are  usually  of  an  approxi- 
mately cylindrical  form,  varying  from  4  to  9  inches  in 
diameter  and  5  to  8  inches  in  depth.  Commonly  the 
whole  section  is  used  with  the  bark  removed,  giving 
blocks  somewhat  irregular  in  shape.  In  some  cases  the 


WOOD    PAVEMENTS.  153 

blocks  are,  however,  cut  to  a  true  cylindrical  form,  the 
sapwood  as  well  as  the  bark  being  cut  away,  by  passing 
them  through  a  set  of  knives  which  are  gauged  to 
turn  out  cylinders  of  given  size.  Several  sets  of  knives 
are  employed,  giving  blocks  of  varying  sizes,  each  block 
being  cut  sufficiently  to  eliminate  all  sapwood  with  as 
little  waste  of  good  material  as  possible.  The  use  of 
sapless  blocks  increases  the  life  of  the  pavement  by 
augmenting  the  resistance  of  the  material  both  to  wear 
of  the  traffic  and  to  the  disintegrating  influences  of  the 
atmosphere  and  moisture. 

Wooden  paving-blocks  other  than  cedar  are  usually 
of  rectangular  form,  8  to  12  inches  long,  3  to  5  inches 
wide,  and  5  to  8  inches  deep.  As  the  blocks  are  usu- 
ally laid  in  courses,  the  width  as  well  as  the  depth  must 
be  constant  for  the  same  work.  It  is  usual  to  cut  the 
blocks  from  plank  of  uniform  thickness, — ordinarily  3 
inches, — as  the  narrow  blocks  give  a  better  foothold  for 
horses  in  damp  weather,  and  also  are  more  easily 
settled  to  a  firm  and  even  bearing  in  the  pavement. 

Hexagonal  blocks  are  also  sometimes  used.  In  a 
mesquite  pavement  constructed  at  San  Antonio,  Texas, 
the  blocks  are  hexagonal  in  form,  the  tops  being 
slightly  smaller  than  the  bottom,  the  diameter  varying 
from  4  to  8  inches,  with  a  depth  of  5  inches. 

The  tendency  of  recent  practice  in  constructing 
wood  pavements  has  been  in  the  direction  of  making 
the  depth  less  than  was  formerly  used.  Five  inches  is 
now  a  very  common  depth. 

Deep  blocks  are  usually  a  waste  of  material,  as  in 
most  cases  not  more  than  2  or  2\  inches  at  most  can 
be  worn  from  the  pavement  before  it  is  replaced,  even 
if  the  traffic  be  sufficient  to  wear  it  out  before  it  rots. 


154     A   TEXT-BOOK   ON    ROADS   AND    PAVEMENTS. 

In  the  use  of  the  Australian  hard  woods  in  London 
a  less  depth  of  block  has  been  tried  and  found  satis- 
factory, and  a  depth  of  about  3^  inches  is  recommended 
by  some  of  the  engineers. 

ART.  64.    FOUNDATIONS  FOR  WOOD  PAVEMENTS. 

The  foundation  which  has  been  most  commonly  used 
in  the  United  States  for  wood  pavement  is  that  com- 
posed of  a  layer  of  boards  upon  sand,  as  described  in 
Art.  49.  This  foundation  has  been  used  in  a  number 
of  places  with  fairly  good  results,  and,  under  light 
traffic,  where  the  first  cost  of  the  pavement  must  be 
low,  its  use  may  sometimes  be  advantageous.  This 
form  of  construction  has  the  disadvantage  of  being 
less  firm  than  the  others  usually  employed,  as  well  as 
that  of  being  perishable  in  its  nature. 

The  life  of  the  pavement  is  therefore  likely  to  be 
less  than  upon  a  more  firm  and  durable  base,  as  the 
destruction  of  the  surface  of  the  pavement  may  be 
caused  by  the  yielding  of  the  foundation.  There  have 
been  instances  recorded  where  a  base  of  this  kind  has 
worn  out  two  or  more  surface  layers,  and  there  are 
others  in  which  the  failure  of  the  foundation  planks  is 
the  beginning  of  the  destruction  of  the  surface.  Much 
depends  upon  the  locality  and  the  quality  of  the  work. 

On  the  better  class  of  wood-block  pavements,  a 
foundation  of  concrete  is  usually  employed.  This 
gives  firm  support  to  the  blocks,  and  admits  of 
even  wear  upon  the  surface  of  the  pavement.  A  dur- 
able foundation  also  has  the  advantage  that  when  the 
surface  layer  is  worn  out,  the  pavement  may  be  resur- 
faced without  renewing  the  foundation. 


WOOD   PAVEMENTS.  155 

In  using  a  concrete  foundation,  a  cushion  coat  of 
sand  is  commonly  employed  on  top  of  the  concrete  in 
which  to  bed  the  blocks  in  order  that  they  may  be 
brought  to  an  even  surface.  Sometimes  a  thin  layer 
of  cement  mortar  is  used  in  place  of  the  sand  upon  the 
concrete;  and  in  London  some  pavements  have  been 
constructed  with  a  thin  layer,  about  £  inch,  of  asphalt 
mastic  over  the  concrete,  the  blocks  resting  upon  the 
mastic. 

Broken-stone  and  gravel  bases  are  also  frequently 
employed  under  wood  blocks,  a  layer  of  boards  being 
usually  placed  over  the  broken  stone  to  form  an  even 
bearing  for  the  bases  of  the  blocks,  although  some- 
times the  blocks  are  bedded  in  a  cushion  coat  of  sand, 
as  with  the  concrete  foundation.  In  Duluth,  a  Telford 
foundation  has  been  employed  under  wood,  laid  in  the 
same  manner  as  for  a  broken-stone  road.  A  layer  of 
gravel  was  placed  upon  the  Telford  pavement  and 
rolled  to  a  smooth  surface,  after  which  the  wood  blocks 
were  laid  directly  upon  the  gravel.  The  reason  given 
for  the  application  of  this  foundation  instead  of  con- 
crete in  this  instance  is  that  the  road-bed  is  of  soft 
material,  which  in  many  places  could  not  be  compacted 
by  the  use  of  a  heavy  roller  before  the  placing  of  the 
concrete,  while  the  stones  of  the  Telford  foundation 
are  forced  to  a  firm  bearing  and  give  a  uniform  sup- 
port. This  difficulty  of  a  soft  road-bed  has  been  some- 
times met  in  other  places  by  rolling  a  thin  layer  of 
gravel  or  broken  stone  into  the  surface  of  the  road-bed, 
the  forcing  of  the  stones  into  the  soil  causing  it  to  be- 
come compact  and  firm,  after  which  the  concrete  may 
be  placed  in  the  usual  manner. 

It  is  perhaps  even  more  important  that  the  founda- 


156     A   TEXT-BOOK   ON    ROADS   AND    PAVEMENTS. 

tion  for  a  wooden  pavement  be  firm  and  unyielding 
than  with  other  kinds  of  block  pavement.  Any  small 
motion  due  to  the  flexibility  of  the  base  is  likely  to 
split  the  blocks,  and  if  through  the  yielding  nature  of 
the  foundation  some  of  the  blocks  are  forced  out  of 
surface  so  that  the  surface  becomes  slightly  uneven, 
the  wear  will  be  very  greatly  increased  over  what  would 
be  the  case  if  the  foundation  were  firm  and  immovable. 

ART.  65.    CONSTRUCTION  OF  WOOD  PAVEMENTS. 

In  the  construction  of  pavements  of  cylindrical 
blocks,  or  whole-tree  sections,  as  in  the  common  cedar 
pavement  of  the  United  States,  blocks  of  varying  sizes 
are  usually  employed,  being  set  in  contact  with  each 
other,  the  smaller  blocks  between  the  larger  ones,  in 
such  a  way  as  to  leave  the  spaces  between  blocks  as 
small  as  possible.  With  blocks  varying  from  4  to  8 
inches  in  diameter,  this  arrangement  gives  good  foot- 
hold for  horses,  and  at  the  same  time  reduces  to  a 
minimum  the  wear  due  to  the  blows  caused  by  wheels 
sinking  into  the  joints  where  large  spaces  exist  between 
the  blocks. 

The  common  arrangement  of  cedar  blocks  is  shown 
in  Fig.  23,  which  represents  a  section  of  pavement  as 
ordinarily  constructed  on  a  sand  and  board  foundation. 

In  the  use  of  rectangular  blocks,  the  blocks  are  set 
with  their  longest  dimension  transverse  to  the  length 
of  the  street.  They  are  usually  arranged  in  courses 
across  the  street,  being  placed  close  together  in  the 
courses,  and  arranged  to  break  joints  in  adjoining 
courses.  Between  courses  a  joint  is  usually  made  J  to 
•J-  inch  in  width,  for  the  purpose  of  affording  a  foothold 


WOOD  PAVEMENTS. 


157 


to  horses.  In  the  older  pavements  of  this  character  a 
much  wider  joint  was  employed,  some  as  much  as  an 
inch  in  width,  with  the  idea  that  they  were  necessary 
to  secure  proper  foothold.  Experience  has  shown, 
however,  that  the  wide  joints  are  not  necessary  ;  and  it 


FIG.  23. 

is  now  commonly  agreed  that  where,  as  is  now  com- 
mon, the  blocks  are  limited  to  a  width  of  3  inches,  a 
joint  £  inch  in  width  is  sufficient  for  the  purpose.  As 
in  the  case  of  round  blocks,  durability  is  greater  where 
the  joints  are  as  small  as  possible,  and  the  liability  of 
the  fibres  of  the  blocks  to  spread  is  eliminated. 

The  tendency  in  practice  is  toward  a  continual  dim- 
inution of  the  width  of  the  joint,  and  some  pave- 
ments have  been  constructed  in  London  in  which  the 
courses  of  blocks  are  placed  in  contact  with  each  other. 
These  are  reported  to  have  given  good  results  in  ser- 
vice, and  to  be  advantageous  in  increasing  the  durabil- 
ity of  the  pavement.  In  some  cases,  where  the  pave- 
ments are  laid  with  close  joints,  an  expansion  joint  ^ 
inch  in  width  is  provided  for  every  30  inches  of  length, 
in  order  to  provide  for  the  swelling  of  the  blocks.  In 
other  cases,  however,  this  is  said  to  have  been  omitted 
without  injury  to  the  pavement. 


158     A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

The  method  of  setting  rectangular  blocks  is  shown 
in  Fig.  24,  which  represents  a  wood-block  pavement  on 
a  concrete  foundation,  as  commonly  constructed. 

In  laying  a  pavement  of  this  kind,  a  course  of  blocks 
is  first  set  across  the  street,  and  then  a  strip  of  wood 
of  the  thickness  of  the  joint  is  set  against  the  row  of 
blocks  and  left  until  the  next  course  is  placed,  or 


'ss.tS. './sft.  \  \  \  \  \  S-lyVm/S^^^^f/// / //> /> 
T 


FIG.  24. 

sometimes  spuds  with  heads  of  the  thickness  of  the 
joints  are  driven  to  the  head  in  the  side  of  each  block, 
and  the  next  row  of  blocks  are  set  against  the  spuds. 

Where  blocks  are  set  in  courses  across  a  street,  it  is 
necessary  that  allowance  be  made  at  the  curb  or  gutter 
for  the  expansion  of  the  blocks.  This  is  usually  ac- 
complished by  leaving  an  open  or  sanded  joint  until 
the  blocks  have  done  swelling. 

The  gutter  blocks  are  very  commonly  turned  with 
their  lengths  along  the  street,  and  sometimes  the  course 
next  the  curb  is  left  out  temporarily  and  filled  with 
sand  to  provide  for  swelling. 

Various  methods  are  employed  for  filling  the  joints 
between  the  blocks.  It  has  been  the  common  practice 
in  the  construction  of  the  cheaper  wood  pavements  in 
the  United  States  to  fill  the  joints  with  sand  and  gravel, 
sometimes  with  a  coating  of  tar,  or  in  some  cases  the 


WOOD   PAVEMENTS.  159 

joint  is  partially  filled  with  tar  and  then  completely 
filled  with  sand  or  small  gravel.  The  objection  to  this 
method  is  that  it  does  not  make  an  impervious  joint, 
which  in  a  wood  pavement,  and  especially  with  the 
cheaper  foundations,  is  a  matter  of  the  utmost  im- 
portance. 

The  best  practice  seems  to  be  to  fill  the  lower  part 
of  the  joint  for  about  one  half  the  depth  with  coal-tar 
paving  cement  and  the  upper  part  with  hydraulic  cement 
mortar.  The  cement  mortar  gives  a  harder  wearing 
surface  than  where  the  entire  joint  is  filled  with  pitch. 
It  also  protects  the  pitch  from  the  softening  action  of 
the  sun  in  warm  weather.  Where  very  narrow  joints 
are  employed,  the  greater  wear  of  the  tar  cement  may 
not  be  of  so  much  importance.  Some  engineers  fill  the 
entire  joint  with  coal-tar  cement,  while  others  use  the 
cement  grout  alone.  The  object  in  all  cases  should  be 
to  make  a  road-surface  as  nearly  impervious  to  water 
as  possible. 

The  coal-tar  paving  cement  used  for  the  purpose  of 
filling  joints  is  the  same  as  that  used  for  brick  pave- 
ments. It  is  usually  formed  of  coal-tar  residuum  mixed 
with  creosote  oil  or  with  tar,  but  it  varies  widely  in 
character  in  different  places.  Sometimes  asphalt  is 
used,  or  asphalt  mixed  with  coal-oil,  but  more  fre- 
quently the  name  asphalt  is  incorrectly  applied  to  coal- 
tar  products,  when  it  is  called  asphaltic  cement.  It  is, 
of  course,  always  applied  hot  to  the  pavement. 

When  the  ordinary  coal-tar  cement  filling  is  employed, 
the  joints  are  first  filled  nearly  full  of  sand  or  gravel, 
which  is  rounded  down  with  a  bar,  after  which  the  hot 
cement  is  poured  in  until  the  joint  is  well  filled. 

When  cement  grout   is  used  it  should  consist  of  a 


l6o     A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

rich  mortar,  I  part  Portland  cement  to  2  parts  sand,  or 
I  part  natural  cement  to  I  part  sand. 

In  London,  where  the  wood  blocks  are  set  upon  a 
layer  of  asphalt,  it  is  customary  to  fill  the  lower  part  of 
the  joint  with  melted  asphaltic  cement  and  the  upper 
part  with  cement  grout.  This  method  is  not,  however, 
extensively  employed. 

The  method  of  procedure  in  constructing  a  wood 
pavement  depends  upon  the  kind  of  foundation  em- 
ployed. When  a  concrete  base  is  used,  a  cushion  coat 
of  sand  about  one  inch  in  thickness  is  usually  spread 
evenly  over  the  concrete.  Upon  this  layer  of  sand  the 
blocks  are  set,  close  together  if  round  blocks,  or  in  rows 
as  already  described  if  rectangular  blocks  are  used. 
The  blocks  are  then  rammed  to  a  firm  and  even  bear- 
ing by  the  use  of  heavy  wooden  rammers,  the  joints 
are  filled  with  gravel  and  paving  cement  or  grout,  as 
the  case  may  be,  a  layer  of  gravel  is  spread  over  the 
surface,  and  the  pavement  is  opened  for  traffic.  Most 
of  the  well-known  London  wood  pavements  are  con- 
structed in  this  manner,  the  blocks  being  of  fir  or  deal, 
and  cut  to  a  rectangular  shape,  although,  as  already 
stated,  some  of  the  later  ones  are  built  with  close  joints 
from  which  the  gravel  filling  is  omitted. 

Where  the  sand  and  board  foundation  is  employed, 
the  best  practice  is  probably  represented  by  the  method 
pursued  at  Chicago,  which  is  approximately  as  follows : 
A  layer  of  sand  3  inches  thick  is  placed  upon  the  road- 
bed, which  has  been  compacted  by  rolling.  The  sand 
is  rammed  or  rolled  until  well  compacted,  and  the 
foundation  layer  of  2-inch  hemlock  planks  laid  length- 
wise of  the  street,  resting  at  their  ends  and  middles 
upon  stringers  I  inch  by  8  inches  bedded  firmly  in  the 


WOOD   PAVEMENTS.  l6l 

sand.  Upon  this  foundation  the  cedar  blocks  are  set 
close  together,  the  joints  are  filled  with  small  gravel 
well  rammed,  and  the  pavement  is  then  flooded  with 
hot  tar  cement  so  as  to  fill  the  interstices  in  the  joints. 
A  coating  of  gravel  one  inch  thick  is  then  placed  upon 
the  pavement,  and  traffic  is  allowed  to  come  upon  it. 

In  some  places  these  pavements  are  constructed  with- 
out the  use  of  the  coal-tar  cement,  the  joints  being 
rammed  full  of  sand  and  gravel ;  in  other  cases  the 
blocks  are  set  upon  rolled  sand  and  gravel  without  the 
boards,  the  blocks  being  rammed  into  place  ;  but  other- 
wise the  construction  is  the  same  as  above. 

The  method  of  construction  advisable  for  any  partic- 
ular work  depends  always  upon  the  local  conditions 
and  requirements.  To  make  a  good  wood  pavement 
there  is  necessary  a  solid  foundation,  blocks  of  good 
material,  and  impervious  joints,  and  all  such  work  should 
be  so  constructed  as  to  secure  these  conditions  in  so  far 
as  available  resources  will  admit.  Weak  construction 
always  involves  high  cost  for  maintenance,  and  greater 
expense  in  the  end  than  good  construction. 

ART.  66.    PRESERVATION  OF  WOOD. 

The  most  serious  objection  commonly  raised  to  the 
use  of  wood  pavements  is  that  wood,  being  porous,  ab- 
sorbs moisture  readily,  and  is  thus  both  liable  to  de- 
struction through  decay  and  to  become  injurious  to 
health.  Various  methods  have  therefore  been  pro- 
posed for  rendering  the  blocks  less  pervious  and  more 
durable  by  impregnating  them  with  various  solutions 
which  shall  fill  the  pores  and  act  as  preservatives. 

The  methods  which  have  been  principally 
known  as  Burnettizing,  Kyanizing,  and 


162     A  TEXT-BOOK  ON   ROADS   AND   PAVEMENTS. 

Burnettizing  consists  in  immersing  the  wood  in  a 
solution  of  chloride  of  zinc  until  the  pores  are  filled 
with  the  solution.  This  is  either  done  by  simply  im- 
mersing and  allowing  the  wood  to  gradually  absorb  the 
solution,  or  by  forcing  the  solution  into  the  pores  of 
the  wood  under  pressure.  The  first  method  requires 
considerable  time  :  about  two  days'  immersion  to  each 
inch  of  thickness  is  usually  allowed  in  order  to  admit  of 
the  wood  becoming  saturated  with  the  solution. 

In  kyanizing  a  saturated  solution  of  corosive  sub- 
limate is  used,  and  the  timber  immersed  in  the  solution 
long  enough  for  the  pores  to  become  well  filled. 

Creosoting  consists  in  impregnating  the  wood  with 
the  oil  of  tar  or  creosote.  In  this  process  the  wood  is 
first  thoroughly  dried,  usually  by  heating  it  in  a  kiln,  and 
the  hot  creosote  is  then  forced  in  under  pressure.  The 
method  of  accomplishing  this  varies  in  different  places. 
In  order  to  be  effective  the  process  must  be  thoroughly 
carried  out  and  the  pores  well  filled.  It  is  commonly 
recommended  that  from  8  to  12  pounds  of  creosote 
per  cubic  foot  of  timber  should  be  forced  in,  as  a  mini- 
mum requirement  for  the  softer  woods,  such  as  are  com- 
monly used  in  pavements.  Creosote  has  the  property 
of  destroying  the  lower  forms  of  animal  life,  and  is 
therefore  an  effective  preservative  against  destruction 
through  these  agencies  where  they  exist.  This  method 
is  therefore  often  employed  for  the  preservation  of 
timber  for  subaqueous  construction  in  sea-water. 

All  of  the  above  processes,  when  properly  applied, 
are  effective  in  preventing  decay,  and  therefore  in 
lengthening  the  natural  life  of  the  wood.  They  also 
render  the  wood  practically  impermeable,  and  thus  re- 
move the  objection  to  the  pavement  based  upon  its 


WOOD   PAVEMENTS.  163 

absorbent  nature.  They  do  not,  in  general,  appear  to 
increase  the  resistance  of  the  wood  to  the  wear  of  the 
traffic,  and  in  most  cases  the  advantage  to  be  gained 
seems  so  small  as  to  render  their  economic  value  for 
this  purpose  at  least  doubtful. 

The  economic  advantage  of  using  treated  blocks  is  a 
question  of  the  relative  costs  of  increasing  the  expense 
of  construction  by  using  them  or  of  the  additional  ex- 
pense of  more  frequent  renewals  where  they  would  be 
necessary  without  the  treatment.  The  desirability  of 
the  treatment  in  any  particular  instance  depends  to 
some  extent  upon  the  traffic  to  which  the  pavement  is 
to  be  subjected  as  well  as  upon  the  character  of  the 
material  available  for  the  purpose.  Where  the  traffic 
is  such  as  to  bring  a  considerable  wear  upon  the  pave- 
ment, and  sound,  well-seasoned  blocks  are  to  be  had, 
there  is  usually  little,  if  any,  advantage  in  the  treated 
blocks,  as  the  pavement  will  ultimately  fail  by  the  wear 
of  the  blocks  in  either  case.  Experience  has  shown 
that  in  many  cases  the  untreated  wood  wears  out  in 
the  pavement  before  decay  sets  in,  and  that  the  appli- 
cation of  the  preservative  processes  would  not  prolong 
the  life  of  the  pavement.  This  has  been  the  result  of 
experience  in  London,  where  after  trying  many  differ- 
ent methods  the  consensus  of  opinion  is  against  the 
use  of  preservative  processes.  It  is  claimed,  however, 
by  some  authorities  that  creosoted  blocks  have  been 
shown  in  some  instances  to  give  greater  resistance  to 
wear  than  untreated  blocks  under  the  same  traffic  (see 
London  Engineering  for  July  29,  1892).  This  London 
traffic  is  heavy,  the  material  well  selected,  and  the 
wear  severe.  There  may  frequently  be  cases,  however, 
wherewith  lighter  traffic  or  with  wood  of  a  less  durable 


164     A   TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

nature  or  less  well  seasoned  the  application  of  preser- 
vatives may  effect  such  a  lengthening  of  the  life  of  the 
pavement  as  to  make  their  application  economically 
desirable. 

In  the  treatment  of  the  wood  it  is  essential  that  the 
process  be  very  thoroughly  applied  in  order  to  get 
good  results.  The  process  most  commonly  recom- 
mended is  creosoting,  and  in  order  to  derive  any  bene- 
fit from  the  treatment  it  is  necessary  that  the  pores  of 
the  wood  be  thoroughly  filled  with  the  oil.  Merely 
dipping  the  blocks  in  creosote  or  tar,  as  is  sometimes 
done,  is  more  likely  to  be  an  injury  than  a  benefit,  and 
has  been  found  in  some  cases  to  be  the  cause  of  decay 
by  closing  the  pores  upon  the  surface  of  the  block 
and  inducing  an  internal  dry-rot.  It  is  also  essential 
to  success  with  creosote  that  the  blocks  be  thoroughly 
dried  before  injecting  the  creosote. 

ART.  67.    MAINTENANCE  OF  WOOD  PAVEMENTS. 

The  ordinary  maintenance  of  wood  pavements,  like 
that  of  most  other  pavements,  consists  in  keeping  the 
pavement  clean  and  in  repairing  from  time  to  time  any 
small  breaks  that  may  appear  in  the  surface  due  to  im- 
perfect material  or  to  the  settling  of  the  foundation. 
These  repairs  would,  of  course,  include  the  removal  of 
any  defective  blocks  and  the  taking  up  and  replacing  of 
any  portion  which  may  settle  out  of  surface  through 
inefficient  support. 

It  is  generally  agreed  that  the  wear  of  a  wood  surface 
is  improved  by  giving  it  an  occasional  coating  of  small 
gravel,  in  some  cases  two  or  three  times  a  year,  and 
permitting  it  to  be  ground  into  the  surface  for  a  few 


WOOD   PAVEMENTS.  165 

days.  It  is  an  advantage  also  that  the  surface  be  kept 
sprinkled  in  warm  weather. 

When  the  wood  pavement  needs  renewal  or  exten- 
sive repairs  the  surface  may  be  relaid  as  with  any  other 
block  pavement:  if  a  permanent  foundation  be  em- 
ployed, by  stripping  the  blocks  from  the  foundation 
and  placing  a  new  surface  in  the  same  manner  as  the 
first  one  ;  with  a  board  foundation  that  also  must  be 
relaid. 

If  the  pavement  is  cut  through  for  any  cause  the 
surface  may  be  replaced  with  the  same  facility  as  other 
block  pavements ;  but  where  a  board  foundation  is 
used  it  is  necessary  to  use  care  in  replacing  in  order  to 
secure  proper  bond  with  the  remainder  of  the  founda- 
tion and  prevent  any  subsequent  settlement  at  the 
line  of  cut.  In  such  cases  it  is  also  necessary  to  com- 
pact the  earth  very  carefully  in  replacing  it,  that  there 
may  be  no  subsequent  settlement. 

The  cost  of  keeping  a  wood  pavement  in  order,  as 
with  any  other  pavement,  depends  upon  the  character 
of  the  work  done  in  construction, — the  better  the  pave- 
ment the  cheaper  the  maintenance. 

ART.  68.    HEALTHFULNESS  OF  WOOD  PAVEMENTS. 

The  use  of  wood  pavement  is  very  often  objected  to 
upon  the  ground  that  it  is  unhealthful  and  likely  to 
give  rise  to  disease.  This  is  based  upon  the  fact  that 
the  material  of  the  pavement,  being  porous  and  ab- 
sorptive of  moisture,  is  likely  to  become  saturated  with 
organic  matter  from  the  foul  liquids  of  the  surface 
soaking  through  it.  This  foul  matter  must  also,  on 
account  of  the  permeable  nature  of  the  material,  pass 


1 66     A   TEXT-BOOK   ON    ROADS   AND    PAVEMENTS. 

to  some  extent  through  the  pavement  and  contami- 
nate the  foundation  and  soil  beneath  as  well,  especially 
where  the  foundation  is  a  permeable  one,  as  in  the  case 
of  boards  or  sand  and  gravel.  In  addition  to  the 
danger  due  to  the  permeability  of  the  wood,  unhealth- 
fulness  may  also  be  caused  by  the  liability  to  decay 
of  the  material. 

There  is  much  difference  of  opinion  among  author- 
ities concerning  the  extent  of  the  danger  to  health 
offered  by  ordinary  wood-block  pavements,  some  re- 
garding the  danger  as  a  very  serious  one  and  protest- 
ing against  the  use  of  wood  in  any  case,  while  others, 
although  admitting  the  permeability  and  perishable 
nature  of  the  material,  consider  its  proper  use  quite  a 
safe  one  and  the  danger  as  somewhat  visionary.  Health 
statistics  of  cities  using  wood  pavements  to  a  large 
extent,  compared  with  those  of  cities  not  using  them,  do 
not  indicate  anything  unfavorable  to  them  ;  but  it  may 
properly  be  said  that  such  statistics  can  seldom  be 
compared  in  such  a  manner  as  to  give  a  reliable  index, 
as  there  are  so  many  other  circumstances  which  may 
affect  public  health,  and  the  conditions  other  than  the 
pavements  are  rarely  the  same. 

The  likelihood  of  a  pavement  producing  unsanitary 
conditions  depends  very  largely  upon  climatic  and  local 
conditions  and  upon  the  construction  of  the  pavement. 
The  opinions  of  those  observing  the  matter  are  there- 
fore usually  based  upon  their  own  local  surroundings. 
Instances  are  recorded  where  blocks  of  wood  after  con- 
siderable service  in  pavements  have  been  found  to  be 
but  little,  if  any,  affected  by  the  absorption  of  the  street 
refuse,  and  the  foundations  to  be  quite  unaffected  by  it ; 
and  in  other  instances  blocks  have  been  found  to  be 


WOOD   PAVEMENTS.  l6/ 

considerably  contaminated,  and  the  foundations  and 
subsoil  saturated  with  filth.  In  most  cases  these  differ- 
ences may  be  attributed  to  differences  in  methods  of 
construction  as  well  as  in  material  used,  and  the  cause 
of  subsoil  contamination  appears  usually  to  be  open- 
joint  construction  rather  than  permeable  blocks. 

Where  good  drainage  exists  and  a  pavement  is  con- 
structed of  sound,  well-seasoned  blocks  with  close,  im- 
pervious joints,  so  that  it  cannot  get  wet  at  the  base, 
the  danger  from  saturation  and  decay  is  probably 
small,  and  on  the  score  of  health  such  a  pavement  is 
much  to  be  preferred  to  a  stone-block  pavement  with 
open  joints.  Under  the  reverse  of  these  conditions  a 
wood  pavement  may  be  a  serious  menace  to  health. 

In  close,  damp  places,  or  climates  giving  the  same 
conditions,  the  liability  to  decay  is  much  greater  than 
where  the  pavement  is  exposed  to  the  sun  and  air. 


CHAPTER   X. 
STONE-BLOCK  PAVEMENTS. 

ART.  69.    STONE  FOR  PAVEMENTS. 

STONE-BLOCK  pavements  are  commonly  employed 
where  the  traffic  is  heavy  and  a  material  needed  which 
will  resist  well  under  wear. 

Stone  for  this  purpose  must  possess  sufficient  hard- 
ness to  resist  the  abrasive  action  of  wheels.  It  must 
be  tough,  in  order  that  it  may  not  be  broken  by  shocks. 
It  should  be  impervious  to  moisture  and  capable  of 
resisting  the  destructive  agencies  of  the  atmosphere 
and  of  weather  changes. 

Experience  only  can  determine  the  availability  of 
any  particular  stone  for  this  use.  The  stone  may  be 
tested  in  the  same  manner  as  brick,  and  perhaps  some- 
thing predicated  as  to  the  probability  of  its  wearing 
well  under  traffic ;  but  the  conditions  of  the  use  of  the 
material  in  the  pavement  are  quite  different  from  those 
under  which  it  may  be  tested,  and  any  tests  looking  to 
a  determination  of  its  weathering  properties  are  apt  to 
be  misleading. 

Examination  of  a  stone  as  to  its  structure,  the  close- 
ness of  grain,  homogeneity,  etc.,  may  assist  in  forming 
an  idea  of  its  nature  and  value  for  wear.  Observations 
of  any  surfaces  which  may  have  been  exposed  for  a 
considerable  time  to  the  weather,  either  in  structures 

168 


STONE-BLOCK   PAVEMENTS.  169 

or  in  the  quarry,  will  be  the  most  efficient  method  of 
forming  an  opinion  concerning  the  weathering  proper- 
ties of  the  stone.  The  conditions  of  use  in  pavements 
are,  however,  somewhat  different  from  ordinary  expos- 
ure in  structures,  on  account  of  the  material  in  the 
pavement  being  subject  to  the  action  of  water  contain- 
ing acids  and  organic  substances  due  to  excretal  and 
refuse  matter.  A  low  degree  of  permeability  usually 
indicates  that  a  material  will  not  be  greatly  affected  by 
these  influences  and  also  that  the  effect  of  frost  will 
not  be  great. 

Granite  and  sandstones  are  commonly  employed  for 
paving  blocks  and  furnish  the  best  material.  Lime- 
stones are  sometimes  used,  but  have  seldom  been  found 
satisfactory.  Trap-rock  and  the  harder  granites,  while 
answering  well  the  requirements  as  to  durability  and 
resistance  to  wear,  are  objectionable  on  account  of 
their  tendency  to  wear  smooth  and  become  slippery 
and  dangerous  to  horses.  Granite  or  syenite  of  a  tough 
homogeneous  nature  is  probably  the  best  material  for 
the  construction  of  a  durable  pavement  for  heavy 
traffic.  Granites  of  a  quartzy  nature  are  usually  brittle 
and  do  not  resist  well  under  the  blows  of  horses'  feet  or 
the  impact  of  vehicles  on  a  rough  surface.  Those  con- 
taining a  high  percentage  of  felspar  are  likely  to  be 
affected  by  atmospheric  agencies,  while  those  in  which 
mica  predominates  wear  rapidly  on  account  of  their 
laminated  structure. 

Sandstones  of  a  close-grained  compact  nature  often 
give  very  satisfactory  results  under  heavy  wear.  They 
are  less  hard  than  granite  and  wear  more  rapidly,  but 
do  not  become  so  smooth  and  slippery,  and  commonly 
form  a  pavement  that  is  more  satisfactory  from  the 


I/O     A   TEXT-BOOK    ON   ROADS   AND   PAVEMENTS. 

point  of  view  of  the  user.  Sandstones  differ  very 
widely  in  character,  their  value  depending  chiefly  upon 
the  nature  of  the  cementing  material  which  holds  them 
together.  In  order  that  a  stone  may  wear  well  and 
evenly  in  a  pavement  it  is  desirable  that  it  be  fine- 
grained, dense,  and  homogeneous,  as  well  as  cemented 
by  a  material  which  is  not  brittle  and  is  nearly  imper- 
vious to  moisture.  Those  sandstones  in  which  the 
cementing  material  is  of  an  argillaceous  or  calcareous 
nature  are  apt  to  be  perishable  when  exposed  to  the 
weather.  The  Medina  sandstones  of  Western  New 
York  and  Ohio  have  been  quite  extensively  used  for 
paving  purposes  and  prove  a  very  satisfactory  material 
for  such  use. 

Limestone  has  not  usually  been  successful  in  use  for 
the  construction  of  block  pavements  on  account  of  its 
lack  of  durability  against  atmospheric  influences.  The 
action  of  frost  commonly  causes  weakness  and  shiver- 
ing, which  produces  uneven  and  destructive  wear  under 
traffic.  There  are,  however,  as  wide  variations  in  the 
characteristics  of  limestones  as  in  those  of  sandstones, 
and  there  may  be  possible  exceptions  to  the  rule  that 
in  general  limestone  is  not  a  desirable  material  for 
block  pavement 

ART.  70.    COBBLESTONE  PAVEMENTS. 

Cobblestones  have  in  the  past  been  quite  extensively 
used  in  the  construction  of  street  pavements,  although 
at  the  present  time  they  have  been  for  the  most  part 
abandoned,  excepting  where  they  are  used  at  the  sides 
of  other  pavements  for  gutter  construction  or  some- 
times between  the  rails  of  a  horse-car  track.  This 


STONE-BLOCK   PAVEMENTS.  17 1 

pavement  as  ordinarily  constructed  is  a  cheap  one  in 
first  cost,  and  it  affords  a  good  foothold  for  horses.  It 
is  not  usually  a  durable  pavement  as  the  stones  are 
easily  loosened  from  their  positions,  although  the 
stones  themselves  may  be  practically  indestructible 
and  used  again  and  again  in  reconstructing  the  surface. 

Cobblestone  pavements  as  commonly  constructed 
are  also  objectionable  because  they  are  permeable  to 
water  and  difficult  to  clean.  They  therefore  collect 
and  become  saturated  with  the  filth  of  the  street 
and  are  very  liable  to  injury  from  frost.  They  are 
also  extremely  rough  and  unsatisfactory  in  use  for 
travel. 

For  paving  the  side-gutters,  where  broken-stone  or 
sometimes  where  wood  is  used  for  the  travelled  portion 
of  the  street,  cobblestones  may  often  be  convenient 
and  useful,  and  form  a  cheap  and  satisfactory  means  of 
disposing  of  surface  drainage.  Such  an  arrangement 
is  shown  in  Fig.  31  (p.  186). 

Cobble  pavements  may  also  sometimes  be  advanta- 
geously used  upon  steep  grades  where  traffic  is  neces- 
sarily slow  and  the  foothold  afforded  becomes  a  very 
important  matter.  When  used  for  this  purpose  a  con- 
crete foundation  should  be  employed  and  the  stones 
be  firmly  bedded  to  prevent  displacement  through  the 
efforts  of  horses  to  obtain  foothold. 

Cobblestones  as  used  for  pavements  are  usually 
rounded  pebbles  from  3  to  8  inches  in  diameter.  They 
are  set  on  end  in  a  layer  of  sand  or  gravel,  rammed  into 
place  until  firmly  held  in  position,  and  then  covered 
with  sand  or  fine  gravel  and  left  to  the  action  of  travel, 
which  soon  works  the  upper  layer  of  sand  into  the  in- 
terstices between  the  stones. 


172    A  TEXT-BOOK  ON  ROADS  AND  PAVEMENTS. 

ART.  71.    BELGIAN  BLOCKS. 

Belgian  block  is  the  name  commonly  applied  to  a 
pavement  formed  of  nearly  cubical  blocks  of  hard  rock. 
In  the  vicinity  of  New  York  this  pavement  has  been 
largely  used,  the  material  being  trap-rock  from  the 
valley  of  the  lower  Hudson.  The  blocks  are  usually 
from  5  to  7  inches  upon  the  edges,  with  nearly  parallel 
faces,  and  as  commonly  laid  are  placed  upon  a  founda- 
tion layer  of  sand  or  gravel  about  6  inches  thick.  This 
shape  of  block  is  objectionable  on  account  of  the  width 
between  joints  being  too  great  to  afford  good  foothold 
to  horses.  The  materials  of  which  Belgian  blocks  have 
ordinarily  been  formed  are  very  hard  and  (as  already 
noted  in  Art.  69)  wear  smooth  in  service,  becoming 
slippery  and  thus  increasing  the  effect  of  the  too  wide 
block.  It  is  also  better  to  have  the  length  of  the 
blocks  somewhat  greater  across  the  street  and  let 
them  break  joints  in  that  direction  in  order  that  they 
may  give  greater  resistance  to  displacement  under  pass- 
ing wheel-loads. 

The  older  pavements  of  this  character  were  usually 
placed  upon  a  sand  foundation.  More  recently,  this 
practice  has,  in  the  better  class  of  work,  been  super- 
seded by  a  more  solid  construction,  a  concrete  base 
being  used. 

ART.  72.    GRANITE  AND  SANDSTONE  BLOCKS. 

For  the  construction  of  the  better  class  of  stone- 
block  pavements,  blocks  of  tough  granite  or  sandstone 
are  used,  set,  in  the  best  work,  upon  a  concrete  base, 
although  sometimes  placed  upon  a  foundation  of  sand 
or  gravel. 


STONE-BLOCK   PAVEMENTS.  173 

These  pavements  when  well  constructed  are  about 
the  most  satisfactory  means  yet  devised  for  providing 
for  very  heavy  traffic,  as  they  present  a  maximum 
resistance  to  wear  with  a  fairly  good  foothold  for 
horses,  and  are  much  more  agreeable  in  service  than  the 
old  form  of  rough  pavements.  There  is  still  much  to 
be  desired  in  the  attainment  of  smoothness  and  ab- 
sence of  noise,  and,  as  a  general  thing,  it  may  be  said 
that  pavements  of  this  kind  are  desirable  only  where 
the  weight  of  traffic  is  so  great  that  the  smoother  pave- 
ments would  not  offer  sufficient  resistance  to  wear. 
Even  in  such  cases  it  may  frequently  be  questionable 
whether  an  additional  expense  for  maintaining  a  pave- 
ment which  would  be  more  pleasant  in  use  and  less 
objectionable  to  occupants  of  adjoining  premises 
would  not  be  advisable  from  an  economical  as  well  as 
from  an  aesthetic  point  of  view. 

Blocks  for  stone  pavements,  in  the  best  work,  are  cut 
in  the  form  of  parallelepipeds,  9  to  12  inches  long,  3 
inches  wide,  and  6  or  7  inches  deep.  The  length  should 
be  sufficient  to  permit  the  blocks  to  break  joints  across 
the  street.  The  width  should  be  less  than  that  of  a 
horse's  hoof  in  order  that  the  joints  in  the  direction  of 
travel  may  be  close  enough  together  to  prevent  a  horse 
from  slipping  in  getting  a  foothold.  The  depth  should 
be  sufficient  to  give  a  bearing  surface  in  the  joints 
large  enough  to  prevent  the  blocks  from  tipping  when 
the  load  comes  upon  one  end  of  it. 

ART.  73.    CONSTRUCTION  OF  STONE-BLOCK 
PAVEMENTS. 

Stone-block  pavement  for  durable  and  £ff.™2™£ 
service  should  be  placed  upon  very  firm  foundations. 


?yy*  ~ 


174     A  TEXT-BOOK   ON  ROADS  AND   PAVEMENTS. 

Bases  of  concrete  are  usually  employed  and  give  the 
best  results.  These  foundations  are  formed  as  de- 
scribed in  Art.  47,  and  consist  of  a  layer  of  concrete  4  to 
8  inches  thick,  6  inches  being  the  most  common  depth. 

In  constructing  the  pavement,  a  cushion  coat  of  sand, 
usually  about  an  inch  thick,  is  spread  upon  the  base  of 
concrete,  for  the  purpose  of  allowing  the  bases  of  the 
paving  blocks  to  be  firmly  bedded  when  the  tops  are 
brought  to  an  even  surface,  the  sand  readily  adjusting 
itself  so  as  to  fill  all  the  spaces  beneath  the  blocks  and 
to  offer  a  uniform  resistance  to  downward  motion  in 
every  part  of  the  pavement,  and  in  like  manner  trans- 
mitting the  loads  which  come  upon  the  pavement  to 
the  foundation  so  as  to  evenly  distribute  them  over  the 
surface  of  the  concrete.  The  sand  used  for  this  pur- 
pose should  be  clean  and  dry,  and  all  large  particles 
sifted  out,  as  they  may  prevent  the  blocks  adjusting 
themselves  properly.  A  thin  layer  of  asphaltic  cement 
is  sometimes  used  in  place  of  the  sand  with  very  good 
results. 

The  blocks  should  be  laid  as  close  together  as  possi- 
ble in  order  to  make  the  joints  small.  They  are  laid, 
like  brick,  with  the  longest  dimension  across  the  street, 
and  arranged  in  courses  transverse  to  the  street,  with 
the  stone  in  consecutive  courses  breaking  joints,  In 
laying,  it  is  considered  best  to  begin  the  courses  at  the 
gutters  and  work  toward  the  middle,  the  crown-stone 
being  required  to  fit  in  tight. 

After  the  blocks  are  placed  they  are  well  rammed  to 
a  firm  unyielding  bearing  and  an  even  surface.  Stones 
that  sink  too  low  under  the  ramming  must  be  taken 
out  and  raised  by  putting  more  sand  underneath. 

As  in  the  case  of  other  block  pavements,  those  of 


STONE-BLOCK   PAVEMENTS.  17$ 

stone  should  be  made  as  impervious  to  moisture  as 
possible.  The  foundation  should  be  kept  dry  and 
moisture  prevented  from  penetrating  beneath  the 
blocks,  where  it  has  a  tendency  to  cause  unequal  settle- 
ment under  loads,  or  disruptions  under  the  action  of 
frost.  In  the  better  class  of  work,  therefore,  the  joints 
are  filled  with  an  impervious  material  which  cements 
the  blocks  together.  Coal-tar  paving  cement  is  com- 
monly employed  for  this  purpose,  as  with  brick  and 
wood,  and  seems  the  most  satisfactory  in  use,  although 
hydraulic  cement  mortar  is  sometimes  used.  The  coal- 
tar  cement  is  commonly  made  by  mixing  coal-tar  pitch 
with  gas-tar  and  oil  of  creosote,  a  proportion  sometimes 
employed  being  100  pounds  pitch,  4  galls,  tar,  and  I 
gall,  creosote. 

The  use  of  cement  between  the  blocks  binds  them 
together  and  increases  the  strength  of  the  pavement  as 
well  as  the  resistance  of  the  blocks  to  being  forced  out 
of  surface.  It  also  deadens  to  some  extent  the  noise 
from  the  passing  of  vehicles  where  asphaltic  or  coal-tar 
cement  is  used. 

The  method  ^of  filling  the  joints  is  usually  to  first 
fill  them  about  one  third  full  of  small  gravel,  then 
pour  in  the  paving  cement  until  it  stands  above  the 
gravel ;  then  another  third  full  of  gravel,  more  cement 
as  before  ;  then  gravel  to  a  little  below  the  top,  and  the 
joint  filled  full  of  cement ;  after  which  a  coating  of 
fine  gravel  is  distributed  over  the  surface. 

Various  modifications  of  the  method  above  outlined 
are  used  in  the  principal  cities  for  a  pavement  to  with- 
stand heaviest  traffic  and  secure  a  maximum  of  dura- 
bility :  essentially  it  represents  the  best  modern  practice. 

A  cheaper  form  of  stone-block  pavement  is  made  by 


176     A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

laying  the  blocks  directly  upon  a  foundation  of  gravel 
or  sand,  either  with  cemented  joints  or  with  joints  filled 
with  gravel  only.  This  gives  a  fairly  good  pavement 
for  streets  of  moderate  traffic,  and  has  been  extensively 
used  in  the  past.  The  present  tendency,  however, 
which  will  probably  increase  in  the  future,  is  to  lessen  the 
use  of  pavements  of  this  character,  and  to  substitute  a 
surface  which  is  more  pleasant  in  use  for  all  service 
where  durability  and  resistance  to  wear  are  not  the 
prime  requisites. 

ART.  74.    STONE  TRACKWAYS. 

In  some  of  the  European  cities,  particularly  in  Italy, 
stone  trackways  are  sometimes  employed  on  streets  of 
heavy  traffic  for  the  purpose  of  diminishing  traction. 
These  trackways  are  formed  of  smooth  blocks  of  stone 
4  to  6  feet  long,  1  8  to  24  inches  wide,  and  6  to  8  inches 
deep,  laid  flat  and  end  to  end  so  as  to  form  a  smooth 
surface  upon  which  wheels  may  move  with  the  least 
possible  resistance.  Between  the  tracks,  and  usually 
the  remainder  of  the  street,  is  commonly  paved  with 
cobble.  The  method  of  construction  is  shown  in  Fig. 
25.  The  tracks  drain  to  the  middle,  and  the  pavement 


FIG.  25. 

between  is  made  concave  and  provided  with  openings 
into  the  storm  sewers  for  the  escape  of  surface-water. 
The  track  and  pavement  are  laid  upon  a  layer  of  sand 
resting  upon  a  broken-stone  or  gravel  foundation. 


STONE-BLOCK   PAVEMENTS.  1/7 

Such  trackways  are  very  durable  under  heavy  traffic, 
and  give  very  light  traction  combined  with  good  foot- 
hold. It  is  possible  that  they  might  advantageously 
be  applied  oftener  than  they  are  on  streets  used  for 
heavy  hauling. 


CHAPTER   XL 
CITY  STREETS. 

ART.  75.    ARRANGEMENT  OF  CITY  STREETS. 

THE  location  of  streets  should  be  planned  with  a 
view  to  giving  direct  and  easy  communication  between 
all  parts  of  a  city.  The  arrangement  should  also  be 
such  as  to  permit  the  subdivision  of  the  area  traversed 
by  them  in  such  a  manner  as  to  give  the  maximum  of 
efficiency  for  business  or  residential  purposes.  The 
most  obvious  and  satisfactory  method  of  accomplishing 
these  purposes  is  usually  by  the  use  of  the  rectangular 
system,  with  occasional  diagonal  streets  along  lines 
likely  to  be  in  the  direction  of  considerable  travel. 

Streets  so  far  as  possible  should  be  systematically 
arranged  and  continuous  throughout  the  extent  of  the 
city,  both  to  facilitate  travel  and  to  admit  of  their  being 
so  named  and  numbered  that  the  locality  of  a  place 
of  business  or  residence  may  at  once  be  evident,  from 
its  address,  to  any  one  familiar  with  the  general  plan 
of  the  city.  The  rectangular  system  is  desirable  on 
this  account,  and  also  because  it  furnishes  blocks  of  the 
best  form  for  subdivision  into  building  lots. 

The  proper  arrangement  of  streets  will  always  neces- 
sarily depend  in  some  measure  upon  the  natural  feat- 
ures of  the  locality,  and  any  system  of  arrangement 
will  be  more  or  less  modified  by  local  topography. 

178 


CITY   STREETS. 

Where  for  topographic  or  aesthetic  reasons  it  may  be 
considered  desirable  to  use  curved  lines  for  the  streets, 
the  continuity  and  uniformity  of  arrangement  should 
be  maintained  as  far  as  possible.  The  use  of  curves 
on  residence  streets  may  sometimes  be  advantageous 
in  reducing  gradients  or  in  its  effect  upon  adjoining 
property  through  avoiding  heavy  earthwork.  Where 
a  change  in  direction  is  necessary  the  use  of  a  curve 
usually  gives  a  better  appearance  than  an  abrupt  bend, 
unless  the  change  can  be  effected  at  the  intersection  of 
a  cross-street.  Care  is  required,  however,  to  prevent 
the  local  introduction  of  curvature  disarranging  the 
general  plans  and  producing  the  chaotic  condition  due 
to  an  irregular  use  of  short  streets. 

In  laying  out  a  rectangular  system  of  streets  the 
blocks  ordinarily  will  preferably  be  long  and  narrow. 
The  distance  needed  between  streets  in  one  direction 
is  only  that  necessary  to  the  proper  depth  of  lots,  while 
in  the  other  direction  the  streets  need  only  be  close 
enough  to  provide  convenient  communication  for  the 
travel  and  traffic.  A  convenient  method  would  be  to 
lay  out  the  main  streets  so  as  to  form  squares  large 
enough  to  permit  the  introduction  of  an  intermediate 
minor  street  through  the  blocks.  These  minor  streets 
may  then  be  introduced  in  the  direction  that  seems 
advisable  in  each  locality.  Such  an  arrangement  is 
shown  in  Fig.  26.  The  diagonal  streets  cut  more  space 
from  the  blocks  traversed  by  them,  but  give  more 
frontage,  and  property  fronting  them  will  usually  have 
more  value  than  other  property  in  its  vicinity. 

The  proper  location  for  diagonal  streets  intended  as 
thoroughfares  for  traffic  is  naturally  determined  by  the 
positions  of  the  business  centres  or  public  buildings 


ISO     A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

and  parks,  from  which  they  may  radiate  in  such  manner 
as  to  bring  the  outlying  portions  of  the  city  into  the 
most  direct  communication  possible. 

A  city  cannot  usually  be  laid  out  complete.     Its  for- 
mation is  a  matter  of  gradual  growth  and  enlargement, 


FIG.  26. 

and  the  end  cannot  be  seen  from  the  beginning.  For 
this  reason  it  is  frequently  necessary  to  undergo  great 
expense  in  the  larger  cities  in  cutting  new  streets  or  in 
changing  the  positions  or  dimensions  of  existing  old 
ones  in  built-up  districts  in  order  to  relieve  the 
crowded  condition  of  the  streets,  which  hampers  busi- 
ness and  renders  travel  difficult  and  unpleasant.  Much 
of  this  difficulty  might  frequently  be  obviated  if  in 
growing  towns  and  cities  proper  attention  were  given 
to  the  regulation  of  suburban  development.  Such  de- 
velopment should  be  under  municipal  control  so  far 
as  to  require  at  least  that  each  new  subdivision  which 
opens  new  streets  should  be  made  with  a  view  to 
affording  proper  ways  of  communication  between  ad- 
joining properties  by  making  streets  continuous. 
Where  such  regulation  does  not  exist  streets  will  be 


r^VT 


1 82     A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

laid  in  any  manner  to  best  develop  the  particular  prop- 
erty in  which  they  are  placed. 

A  good  example  of  the  advantages  of  systematic  and 
liberal  plans  in  street  arrangement,  as  well  as  of  the 
evils  of  unregulated  extension,  is  given  by  the  case  of 
Washington,  D.  C. 

Fig.  27  shows  a  portion  of  the  city  of  Washington 
illustrating  its  systematic  arrangement.  It  consists  of 
a  rectangular  system,  together  with  two  sets  of  diag- 
onal avenues,  and  open  squares  or  circles  at  the  inter- 
sections of  the  avenues. 

Fig.  28  shows  a  number  of  suburban  subdivisions  on 
the  borders  of  the  city  of  Washington,  made  previous 
to  the  adoption  of  the  law  regulating  them.  In  some 


DD5 


DSQE 
DDES 

DDDB 

DDDDH 


FIG.  28. 


cases  the  streets  of  adjoining  subdivisions  have  no 
communication  with  each  other,  and  the  general  ten- 
dency is  toward  a  labyrinth  of  short  streets.  The  law 
now  requires  that  all  street  extension  within  the  Dis- 


CITY    STREETS.  183 

trict  of  Columbia  shall  conform  to  the  general  plan  of 
the  city  of  Washington  ;  and  under  the  operation  of 
this  law  the  lines  of  many  of  the  city  streets  have  been 
extended  to  all  parts  of  the  District,  and  all  of  the 
suburban  development  is  being  gradually  brought  with 
the  city  into  one  harmonious  whole,  on  the  same  gen- 
erous plan  that  exists  within  the  city.  The  rectification 
of  the  irregular  plats  upon  the  borders  of  the  city  must, 
however,  be  a  matter  of  heavy  expense  to  the  District. 

ART.  76.    WIDTH  AND  CROSS-SECTION. 

The  width  of  city  streets  is  important  both  on  ac- 
count of  its  influence  upon  the  ease  with  which  traffic 
may  be  conducted,  and  because  of  its  effect  upon  the 
health  and  comfort  of  the  people,  by  determining  the 
amount  of  light  and  air  which  may  'penetrate  into 
thickly  built-up  districts. 

To  properly  accommodate  the  traffic  of  commercial 
thoroughfares  in  business  districts  of  towns  of  consider- 
able size,  a  street  should  have  a  width  of  100  to  160 
feet,  the  whole  of  it  to  be  used  for  roadway  and  side- 
walks. Wide  streets  are  especially  needed  where,  as  in 
the  larger  cities,  they  are  bordered  by  high  buildings 
or  are  to  carry  lines  of  street  railway. 

Residence  streets  in  a  town  of  considerable  size, 
where  houses  are  set  out  to  the  property  line  and  stand 
close  together,  should  have  a  width  of  at  least  80  to  100 

o 

feet  in  order  to  look  well  and  give  plenty  of  light  and 
air. 

The  streets  in  nearly  all  large  towns  are  laid  out  too 
narrow  ;  they  are  crowded  and  dingy.  The  chief  diffi- 
culty is  that  the  future  of  a  street  is  not  usually  fore- 


1 84     A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

seen  when  it  is  located.  Owners  in  subdividing  prop- 
erty are  only  anxious  to  get  as  many  lots  as  possible 
out  of  it,  and  there  are  usually  no  regulations  looking 
to  the  future  health  and  comfort  of  resident  when  the 
street  shall  be  built  upon.  In  the  growth  of  a  town  the 
nature  of  localities  change :  residence  streets  become 
business  streets,  streets  devoted  to  retail  trade  become 
wholesale  streets,  and  mercantile  districts  are  given  up 
to  manufacturing.  If  a  city  could  be  laid  out  com- 
plete from  the  beginning  it  would  be  comparatively 
easy  to  consider  the  requirements  to  be  met  and  locate 
the  streets  accordingly.  Under  existing  conditions 
this  is  not  possible,  but  a  more  liberal  policy  in  planning 
streets  would  usually  be  found  of  advantage  in  any 
growth  that  may  ensue.  There  is  also  very  frequently 
an  immediate  financial  advantage  in  the  enhancement 
of  values  due  to  wide  streets.  A  lot  100  feet  deep  on 
a  street  80  feet  wide  will  nearly  always  be  of  greater 
value  than  if  the  same  lot  be  no  feet  deep  and  the 
street  only  60  feet  in  width. 

In  Washington,  D.  C,  which  probably  has  the  best 
general  system  of  any  American  city,  no  new  street  can 
be  located  less  than  90  feet  in  width,  and  avenues 
must  be  at  least  120  feet  wide.  Intermediate  streets, 
called  places,  60  feet  wide,  are  allowed  within  blocks, 
but  full-width  streets  must  be  located  not  more  than 
600  feet  apart.  The  value  of  this  liberal  policy  to  the 
city  of  Washington  is  evident  not  only  in  the  increased 
comfort  of  the  people,  but  in  its  large  growth  as  a  resi- 
dential city  and  the  increased  value  of  property  in  it. 

While  it  is  advantageous  to  have  the  street  wide  be- 
tween building-lines,  it  is  not  necessary  that  the  whole 
street  width  be  used  for  pavements.  The  street  pave- 


CITY   STREETS.  185 

ment  should  be  gauged  in  width  by  the  immediate 
necessities  of  the  traffic  which  is  to  pass  over  it.  The 
pavement  should  be  wide  enough  to  easily  accommo- 
date the  traffic,  but  any  unnecessary  width  is  a  tax 
upon  the  community  in  the  construction  and  mainte- 
nance of  more  pavement  than  should  be  required,  and 
perhaps  diminishes  the  length  of  street  which  may  be 
improved  with  available  funds.  Thus,  for  a  residence 
street  in  general  a  width  of  30  to  35  feet  between 
curbs  is  usually  ample,  with  a  foot-walk  upon  each  side 
6  to  10  feet  wide.  The  remainder  of  the  street  width 
should  be  made  into  lawns  upon  each  side,  with  tree 
spaces  between  the  sidewalk  and  roadway. 

Fig.  29  shows  in  partial  section  the  arrangement  of  a 


. 

-  -   -   -  -  --vL.3,*-  -6FV..-*--  UAWN    17  Fh 

FIG.  29. 

QO-ft.  residence  street  for  moderate  traffic.  For  resi- 
dence streets  of  lesser  importance,  where  the  travel 
is  light  and  the  street  is  only  required  to  furnish 
facilities  to  meet  the  needs  of  its  immediate  locality,  a 
less  width  of  pavement  may  often  be  advantageously 
used.  A  pavement  24  feet  wide  is  sufficient  to  accom- 
modate a  very  considerable  amount  of  light  driving, 
and  in  many  places,  especially  in  the  smaller  towns 
where  funds  for  effective  improvement  are  obtained 
with  difficulty,  even  less  widths  may  be  employed  with 
the  result  of  improving  the  streets  both  in  appearance 
and  usefulness.  All  that  is  really  needed  in  such  cases 
is  room  for  teams  to  pass  comfortably  and  to  turn 
without  difficulty.  The  narrowing  of  roadways  on 
streets  of  light  traffic  to  what  is  really 


1  86     A   TEXT-BOOK   ON   ROADS   AND    PAVEMENTS. 

often  make  possible  improvements  which  will  turn  a 
broad  sea  of  mud  into  a  narrow  hard  roadway  and  a 
grass-plat.  Fig.  30  shows  the  arrangement  of  a  village 
street  50  feet  wide  for  light  service. 

In  many  cases  for  village  streets,  where  the  traffic  is 
light  and  it  is  essential  that  the  cost  of  construction  be 


FIG.  30. 

low,  it  may  be  good  practice  to  construct  the  travelled 
portion  of  the  roadway  of  macadam,  wood,  or  other 
pavement,  and  use  cobble  gutters  at  the  sides  without 
curbs.  Fig.  31  shows  a  roadway  30  feet  wide,  with 
macadam  middle  and  cobble  gutters.  In  Saginaw, 
Mich.,  this  method  has  been  followed,  using  either 
macadam  or  wood  blocks  for  the  middle  portion,  and 


FIG.  31. 

in   the  report  of  City  Engineer  Roberts  for  1893  it  is 
recommended  as  economical  and  efficient. 

The  cross-section  of  streets  must  be  arranged  with 
reference  to  proper  surface  drainage.  The  street  is 
given  a  crown  at  the  middle  to  throw  the  water  into 
the  gutters,  and  sidewalks  usually  have  a  sufficient  in- 
clination toward  the  gutter  to  cause  them  to  drain  over 
the  curb.  The  section  necessary  for  street  drainage  is 
discussed  in  Art.  10.  The  street  is  usually  made  prac- 
tically level  across,  the  curbs  and  sidewalks  at  the  two 
sides  being  given  the  same  elevation.  The  parking  at 


CITY   STREETS.  1 87 

the  sides  may  have  a  slope  between  the  sidewalk  and 
the  building-line  when  it  is  necessary  or  advantageous. 
Sometimes,  on  streets  along  a  slope,  expense  may  be 
saved  or  adjoining  property  benefited  by  placing  the 


FIG.  32. 

sidewalk  at  a  different  elevation  from  that  of  the  street, 
as  shown  in  Fig.  5>  or  by  placing  one  curb  lower  than 
the  other  and  moving  the  crown  of  the  road  to  one 
side,  as  shown  in  Fig.  32. 

ART.  77.    STREET  GRADES. 

The  grades  of  city  streets  necessarily  depend  mainly 
upon  the  topography  of  the  site.  Wherever  possible, 
it  is  desirable  that  grades  be  uniform  between  cross- 
streets. 

In  establishing  grades  for  new  streets  through  unim- 
proved property,  they  may  usually  be  laid  with  refer- 
ence only  to  obtaining  the  most  desirable  gradients  for 
the  street  within  a  proper  limit  of  cost.  But  where 
improvements  have  already  been  made,  and  located 
with  reference  to  the  natural  surface  of  the  ground,  it 
is  frequently  a  matter  of  extreme  difficulty  to  give  a 
desirable  grade  to  the  streets  without  injury  to  adjoin- 
ing properties.  In  such  cases  it  becomes  a  question  of 
how  far  individual  interests  shall  be  sacrificed  to  the 
general  good.  It  may  be  said  in  this  connection  that 
adjustments  to  new  grades  are  usually  accomplished 
much  more  easily  than  would  be  anticipated,  and  when 
accomplished  the  possession  of  a  desirable  grade  is  of 


1 88     A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

very  considerable  value  to  adjoining  property.  Too 
great  timidity  should  not,  therefore,  be  felt  in  regard 
to  making  necessary  changes  because  of  the  fear  of  in- 
juring property  in  the  locality. 

Where  a  grade  if  made  continuous  between  inter- 
secting streets  would  be  nearly  level,  it  is  frequently 
necessary  to  put  a  summit  in  the  middle  of  the  block 
and  give  a  light  gradient  downward  in  each  direction 
to  the  cross-streets  in  order  to  provide  for  surface 
drainage.  The  amount  of  slope  necessary  to  provide 
for  proper  drainage  depends  upon  the  character  of  the 
surface  and  smoothness  of  the  gutter.  For  a  surface 
of  earth  or  macadam  the  slope  should  not  be  less  than 
about  I  in  100,  and  for  paved  streets  from  I  in  200  to 

1  in  250. 

In  some  cases  it  may  be  possible  to  give  sufficient 
slope  to  gutters  to  carry  off  the  surface-water  by  mak- 
ing the  gutter  deeper  at  the  ends  than  in  the  middle  of 
the  block  without  making  a  summit  in  the  crown  of 
the  street.  The  curb  in  such  case  would  be  made 
level  or  of  uniform  gradient. 

The  smoother  forms  of  pavement  are  only  applicable 
to  light  gradients.  Rock  asphalt  is  usually  limited  to 

2  or  2^  per  cent  grades.    Trinidad  asphalt  maybe  used 
to  grades  of  about  4  per  cent.     Brick,  if  kept  clean,  is 
safe  on  gradients  of  about  6  per  cent ;  wood,  on  those 
a  little  steeper ;  and  stone   blocks  are  satisfactory  to 
about  a  lo-per-cent  gradient. 

Pavements  on  steeper  gradients  must  be  made  rough 
in  order  to  insure  a  safe  foothold  to  horses.  On  grades 
steeper  than  9  or  10  per  cent  cobblestones  are  prefera- 
ble to  rectangular  stone  blocks,  as  they  give  better 
foothold,  and  the  speed  of  travel  being  necessarily 


CITY   STREETS.  189 

slow  the  roughness  is  of  less  consequence.  For  such 
use  it  is  desirable  to  have  the  cobblestones  set  on  a 
concrete  foundation  and  the  joints  filled  with  paving 
cement  after  the  manner  of  a  first-class  block  pave- 
ment, as  the  wear  on  a  steep  slope  will  be  severe.  Or- 
dinary stone  blocks  may  be  laid  on  steep  streets  with 
wide  joints,  about  an  inch,  so  as  to  give  better  foothold 
than  the  common  form ;  or  the  corners  of  the  stones 
may  be  bevelled  on  the  upper  edges  and  set  in  the  usual 
manner. 

In  a  report  on  the  streets  of  Duluth  in  1890,  Messrs. 
Rudolph  Hering  and  Andrew  Rosewater  recommend 
for  steep  streets,  in  addition  to  the  above,  that  brick 
may  be  used  in  which  the  tops  are  rounded,  and  that 
wood  blocks  for  such  use  have  their  upper  edges  cham- 
fered on  each  side,  or  if  round  blocks  be  used,  around 
the  blocks. 

ART.  78.    STREET  INTERSECTIONS. 

At  intersections  the  crown  of  the  roadway  pavement 
on  each  street  should,  if  possible,  be  continuous  to  the 
centre  of  intersection,  in  order  to  prevent  vehicles  on 
one  street  from  being  subjected  to  the  jar  incident  to 
passing  over  the  gutter  of  the  other.  Where  a  storm 
sewer  is  available  into  which  the  water  from  the  gut- 
ters on  the  upper  side  can  be  emptied  this  is  a  simple 
matter,  but  where  such  sewers  do  not  exist  it  requires 
the  adoption  of  some  special  means  of  draining  the 
gutters  on  the  upper  side.  This  may  sometimes  be 
accomplished  by  a  culvert  across  the  street,  the  gutters 
being  somewhat  depressed  at  the  corners  to  bring  the 
channel  sufficiently  low.  In  other  cases,  where  the 
slope  is  sufficient,  it  is  more  satisfactory  to  construct 


I QO     A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

an  underground  pipe-drain  from  the   upper  corner  to 
some  point  in  the  gutter  below  the  crossing. 

Where  the  rate  of  grade  is  such  that  it  is  feasible,  it 
is  desirable  that  the  grade  of  both  streets  should  be 
brought  to  a  level  at  intersections.  The  top  of  the  curb 
at  the  four  corners  should  be  at  the  same  elevation,  thus 
permitting  the  continuation  of  the  full  section  of  each 
roadway  until  they  intersect.  It  is  also  desirable  that 
the  sidewalks  at  the  corners  be  level ;  that  is,  the  points 
a  a  in  Fig.  33  should  all  be  placed  at  the  same  eleva- 


a 


FIG.  33. 

tion,  which  will  make  the  entire  street  section,  includ- 
ing sidewalks,  horizontal  across  the  direction  of  travel 
on  each  street. 

On  very  steep  slopes  it  may  not  be  possible  to  flatten 
out  the  grade  to  a  level  in  crossing  transverse  streets, 
and  in  such  cases  the  elevations  require  study,  and  need 
to  be  carefully  worked  out  for  each  particular  case.  In 
the  report  of  Messrs.  Rudolph  Hering  and  Andrew 
Rosewater  upon  the  streets  of  Duluth,  it  is  recom- 
mended that  in  all  cases  the  grade  shall  be  reduced  to 
3  per  cent  between  the  curb-lines  of  cross-streets,  and 


CITY   STREETS.  IQI 

the  grade  of  the  curb  reduced  in  all  cases  to  8  per  cent 
for  the  width  of  the  sidewalks  of  intersecting  streets. 
This  is  to  be  considered  the  maximum  allowable  rate 
of  transverse  grade,  and  only  to  be  employed  in  case  of 
necessity.  If  in  Fig.  33  the  arrow  represents  the  direc- 
tion of  steep  slope,  and  the  street  transverse  to  that 
direction  has  a  roadway  40  feet  wide  with  sidewalks 
10  feet  wide,  the  above  limits  would  permit  the  curb 
at  c  to  be  1.2  feet  lower  than  that  at  ^,  and  admit  of 
a  fall  of  0.8  foot  in  the  curb  line  from  a  to  b  and  from 
c  to  d.  If  both  streets  have  the  same  grade  and 
width  the  curb  at  the  lowest  corner  would  be  2.4  feet 
lower  than  at  the  highest  corner. 

Sometimes,  where  the  parallel  streets  in  one  direction 
follow  the  lines  of  greatest  slope,  and  the  cross-street 
are  normal  to  them,  the  proper  grades  at  intersections 
may  be  arranged  by  giving  the  streets  along  the  slope 
a  section  similar  to  that  shown  in  Fig.  32  throughout  its 
length,  thus  permitting  the  street  in  the  direction  of 
slope  to  continue  its  grade  across  the  intersection 
without  altering  at  that  point  the  side  slope  of  the 
cross-street. 

For  a  case  of  maximum  slope  this  would  make  the 
section  of  the  roadway  of  the  cross-street  a  plane  sur- 
face sloping  uniformly  from  the  upper  to  the  lower 
curb,  or  in  Fig.  32  it  would  transfer  the  street  crown  to 
the  upper  curb. 

ART.  79.    FOOTWAYS. 

Footways  are  not  required  to  bear  the  heavy  loads 
which  come  upon  the  roadway  pavement,  but  in  streets 
of  considerable  travel  are  subjected  to  a  continual 


IQ2     A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

abrading  action,  and  for  good  service  are  required  to 
be  of  a  material  which  will  resist  abrasion  well,  of  so 
uniform  a  texture  as  to  wear  evenly,  and  not  hard 
enough  to  become  smooth  and  slippery  in  use. 

A  good  sidewalk  should  always  present  an  even  sur- 
face, and  therefore  requires  a  firm  foundation  to  resist 
the  displacement  of  the  blocks  of  which  it  may  be  com- 
posed. It  must  also  be  durable  under  atmospheric 
changes,  and  of  material  that  may  be  easily  cleaned. 
The  materials  commonly  employed  are  gravel,  wood, 
brick,  tar,  asphalt,  stone,  and  artificial  stone. 

Gravel  walks  are  the  cheapest  of  footways  where 
suitable  material  is  available.  They  are  constructed  in 
a  manner  similar  to  that  used  for  gravel  roadways,  and 
require  that  the  bed  of  the  walk  be  well  drained,  and 
that  it  be  well  compacted  by  rolling  or  ramming  before 
the  walk  is  placed  upon  it.  The  best  gravel  walks  are 
usually  built  upon  a  base  of  rough  stone.  This  base 
may  be  6  or  8  inches  thick,  and  forms  a  solid  founda- 
tion upon  which  the  gravel  surface  may  be  placed  and 
sustained  against  settling.  Walks  constructed  in  this 
manner  are  frequently  used  in  city  parks  where  the 
travel  is  considerable.  On  suburban  roads,  gravel 
walks  usually  consist  of  a  thin  surface  of  gravel  laid 
upon  the  earth-bed,  and  are  replaced  by  some  other 
surface  when  a  more  expensive  construction  can  be 
afforded.  Gutters  are  frequently  necessary  to  protect 
the  walks  from  the  wash  of  surface-water,  which  other- 
wise very  quickly  destroys  it. 

Wood  is  commonly  used  for  walks  in  the  form  of 
planks,  which  are  laid  on  stringers,  the  planks  being 
placed  perpendicularly  to  the  direction  of  travel.  It  is 
comparatively  short-lived,  and  requires  considerable 


CITY   STREETS.  193 

expenditure  for  repairs,  as  the  material  is  perishable 
and  also  wears  rapidly. 

Brick  footway  pavements  have  been  extensively  used 
for  many  years,  and  form,  when  well  constructed,  a  very 
durable  and  satisfactory  sidewalk.  As  commonly  con- 
structed they  consist  of  ordinary  hard-burned  bricks 
laid  flat  upon  a  layer  of  sand  over  the  earth-bed.  For 
light  travel,  pavements  so  constructed  may  last  well 
and  give  good  service  ;  but  they  are  apt  to  suon  become 
uneven  through  the  sinking  of  the  bricks  because  of  in- 
sufficient foundation. 

In  constructing  such  a  pavement  the  sand  layer 
should  be  well  compacted  by  rolling  or  ramming  be- 
fore setting  the  bricks,  which  should  also  be  rammed 
to  a  firm  and  even  bearing.  To  give  satisfactory  re- 
sults, a  foundation  of  sand  and  gravel  or  broken  stone 
should  be  formed  8  or  10  inches  in  thickness.  In 
Washington  a  layer  of  gravel  4  inches  thick  and  well 
compacted  is  used,  with  a  layer  of  sand  of  the  same 
thickness  upon  it  to  receive  the  surface.  In  forming 
the  pavements,  the  bricks  are  laid  flat  and  as  close  as 
possible.  The  joints  are  filled  with  sand,  usually  by 
coating  the  surface  with  a  layer  of  sand  before  ram- 
ming, and  after  completion  a  second  coating,  which  is 
allowed  to  remain  a  few  days  after  admitting  the  travel 
to  it. 

Care  must  be  used  in  selecting  brick  for  this  purpose 
to  get  only  hard-burned  brick  of  uniform  quality, 
in  order  that  the  resistance  to  wear  may  be  even. 
The  use  of  vitrified  paving  brick,  as  used  for  roadway 
pavement,  would  be  of  advantage  on  walks  subjected 
to  heavy  wear. 

The  use  of  a  concrete  foundation  and   setting  the 


194     A  TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

brick  on  edge  and  in  mortar,  after  the  manner  of  con- 
structing a  roadway  pavement,  makes  a  very  durable 
sidewalk  under  heavy  travel.  It  is,  however,  some- 
what expensive,  and  usually  a  stone  surface  would  be 
preferable  where  such  expense  is  to  be  incurred. 

Footway  pavements  of  a  concrete  in  which  coal-tar 
is  the  binding  material  have  been  widely  used,  but  have 
not  usually  been  satisfactory  in  use.  As  commonly 
constructed  they  wear  rapidly  and  soften,  becoming 
very  disagreeable  in  hot  weather.  Some  pavements  of 
this  character  have,  however,  shown  fairly  good  service. 

Numerous  methods  have  been  proposed  and  tried 
for  the  construction  of  tar  foot-walks,  differing  from 
each  other  in  the  materials  mixed  with  the  tar  to  form 
the  concrete,  and  in  the  manipulation  of  the  process. 
Ashes  mixed  with  sand  and  gravel  are  usually  em- 
ployed, and  sometimes  clinkers  from  an  iron  foundry. 
A  somewhat  successful  pavement  of  this  class  has  a 
small  amount  of  Portland  cement  mixed  with  the  ashes 
and  sand  used  in  forming  the  concrete  before  the  addi- 
tion of  the  tar. 

Asphalt  footway  pavements  are  formed  either  of  as- 
phalt blocks  or  of  a  surface  of  sheet  asphalt.  Where 
blocks  are  used  they  are  laid  in  the  same  manner  as 
brick  upon  a  foundation  of  sand  or  gravel.  The 
blocks,  or  tiles  as  they  are  commonly  called,  are  usu- 
ally made  flat,  about  8  inches  square  and  2  to  2^  inches 
thick.  They  are  laid  with  their  edges  either  at  right 
angles  to  the  street  line  or  at  an  angle  of  45°  with 
the  street  line, — usually  at  right  angles,  on  account  of 
greater  ease  in  laying. 

Sheet-asphalt  footways  are  laid  in  the  same  manner 
as  an  asphalt  street  pavement,  the  pavement,  however, 


CITY   STREETS.  195 

being  given  a  less  thickness.  In  Washington,  D.  C., 
these  pavements  are  made  about  3  inches  thick,  and 
constructed  upon  a  bituminous  base.  Material  re- 
moved from  street  pavements  in  resurfacing  is  used  for 
forming  the  surface  material  of  the  footway.  Mixtures 
of  coal-tar  and  asphalt  similar  to  that  used  for  distillate 
pavement,  as  noted  in  Art.  61,  are  also  used  in  foot- 
ways, and  are  commonly  spoken  of  as  asphalt. 

In  the  use  of  rock  asphalt  for  footways,  the  asphalt 
mastic  mentioned  in  Art.  57  is  commonly  used,  mixed 
with  sand  or  gravel  to  give  a  wearing-surface.  The  in- 
gredients are  heated  together  and  applied  hot  to  a 
broken-stone  or  concrete  foundation.  In  Europe  hy- 
draulic cement  concrete  is  used  for  the  base,  as  in  the 
driveways.  A  layer  of  3  or  4  inches  of  concrete  is  em- 
ployed, with  a  surface  layer  of  rock  asphalt  or  asphalt 
mastic  and  sand,  J  to  f  inch  in  thickness,  for  ordinary 
work. 

Natural  stone  for  foot-walks  is  ordinarily  used  in  the 
form  of  flagging.  Where  flagstones  of  proper  size  and 
good  wearing  qualities  may  be  readily  obtained,  this 
kind  of  pavement,  if  well  laid,  makes  a  durable  and 
satisfactory  foot-walk.  Flagstones  should  be  set  upon 
a  solid  foundation  and  be  firmly  bedded  so  as  to 
preserve  an  even  surface.  They  should  not  be  laid,  as 
is  common  in  many  places,  directly  upon  an  earth-bed, 
but  should  have  a  cushion  layer  of  sand  or  of  some 
porous  material  to  prevent  unequal  settling  under  the 
action  of  frost. 

Artificial-stone  pavements,  when  well  constructed  of 
good  materials,  make  the  most  satisfactory  of  foot- 
ways. They  form  an  even  surface,  quite  agreeable  in 
service,  and  are  durable  and  economical  where  exposed 


196     A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

to  considerable  travel.  Pavements  of  this  kind  are 
either  constructed  of  blocks  of  material  made  at  a  fac- 
tory and  carried  to  the  site  of  the  walk,  or  the  stone  is 
formed  in  the  position  in  which  it  is  to  be  used.  The 
latter  plan  is  more  commonly  followed  and  admits  of 
the  use  of  larger  blocks,  the  size  in  this  case  being  only 
limited  by  the  necessity  of  providing  for  changes  of 
dimension  with  those  of  temperature,  very  large  blocks 
being  liable  to  crack  under  such  changes. 

There  are  a  number  of  methods  of  preparing  arti- 
ficial stone  for  pavements,  many  of  them  patented, 
differing  to  some  extent  in  the  composition  of  the 
material  or  the  details  of  the  work.  In  general  the 
process  consists  in  placing  a  layer  of  concrete  4  to  6 
inches  thick  upon  a  layer  of  gravel  or  other  porous 
material.  A  surface  of  rich  mortar  or  concrete,  com- 
posed of  hydraulic  cement  with  sand  or  crushed 
granite,  is  given  to  the  pavement,  and  the  surface  is 
commonly  roughened  by  scratching  lines  upon  it 
before  it  is  hardened.  As  with  all  other  concrete- 
work,  the  pavement  needs  to  be  kept  damp  and  pro- 
tected from  the  sun  until  the  mortar  is  fully  set.  A 
layer  of  damp  sand  spread  over  the  surface  may  be 
advantageously  employed  to  protect  it  for  several  days 
after  it  is  opened  to  travel. 

ART.  80.    CURBS  AND  GUTTERS. 

Curbs  are  usually  set  in  the  streets  of  towns  at  the 
sides  of  roadway  pavements  for  the  purpose  of  sus- 
taining and  protecting  the  sidewalk  or  tree  space,  and 
of  forming  the  side  of  the  gutter.  They  are  commonly 
formed  of  natural  stone,  but  sometimes  also  of  artificial 
stone,  clay  blocks,  or  cast  iron. 


CITY   STREETS.  197 

The  curbs  used  in  different  places  vary  considerably 
in  form  and  dimensions.  Stone  curbs  vary  from  4  to 
12  inches  in  width  and  from  8  to  24  inches  in  depth. 
They  are  usually  employed  from  3  to  6  feet  in  length, 
and  set  with  close  joints. 

The  depth  must  be  sufficient  to  admit  of  their  being 
firmly  bedded,  and  to  prevent  overturning  into  the 
gutter.  The  front  of  the  curb  should  be  hammer- 
dressed  to  a  depth  greater  than  its  exposure  above  the 
gutter,  and  the  back  deep  enough  to  permit  the  side- 
walk pavement  to  fit  close  against  it  where  the  side- 
walk adjoins  the  curb.  The  ends  of  the  blocks  should 
also  be  dressed  to  the  depth  of  exposure,  and  the  part 
below  the  ground  trimmed  off  so  as  to  permit  the 
dressed  ends  to  come  in  contact  when  laid. 

Granite  is  usually  considered  the  best  material  for 
curbs,  although  both  sandstones  and  limestones  are 
used  in  many  places.  In  the  vicinity  of  New  York  the 
North  River  bluestone  has  proved  a  good  material  for 
the  purpose. 

There  are  various  ways  of  setting  the  curb.  The 
object  should  be  to  bed  it  firmly  on  a  solid  foundation. 
The  best  method  is  to  place  a  bed  of  concrete  under  it. 
This  construction  is  shown  in  Fig.  34,  which  repre- 
sents the  method  used  in  setting  granite  curb  in  Wash- 
ington, D.  C.  The  curb  is  held  firmly  in  place  by  the 
concrete  foundation,  which  joins  it  rigidly  to  the  road- 
way pavement. 

Where  the  concrete  foundation  is  not  used  under 
the  curb  a  deeper  curbstone  is  necessary,  usually  from 
1 8  to  24  inches  in  good  work.  Curbs  are  very  com- 
monly set  in  the  natural  ground,  the  pavement  coming 
against  it  on  one  side ;  but  it  is  usually  found  advan- 


198     A   TEXT-BOOK   ON   ROADS   AND    PAVEMENTS. 

tageous  to  lay  them   upon  a  bed  of  gravel  or  broken 
stone,  with  gravel  filled  in   the  trench  about    them. 


FIG.  34. 

The  ordinary  method   of   setting   curbs    is   shown  in 

Fig.  35- 

The  Washington  specifications  for  ordinary  work 
require  that  a  bed  of  gravel  4  inches  deep  be  used 
under  the  curb,  and  that  the  trench  be  filled  with 


FIG.  35. 


gravel  placed  in  layers  3  or  4  inches  deep,  each  layer 
being  thoroughly  rammed  before  adding  the  next. 
Curbs    of   artificial    stone    or    concrete    are    usually 


CITY   STREETS.  199 

formed  by  mixing  the  concrete  upon  the  ground  and 
placing  it  in  the  position  it  is  to  occupy,  using  a  board 
mould,  as  in  constructing  artificial-stone  foot-walks,  to 
give  it  proper  shape.  By  this  method  of  construction 
the  curb  and  gutter  may  be  made  practically  in 
one  piece^  where  a  concrete  base  is  used  for  the 
pavement.  The  concrete  for  the  curb  and  gutter  is 
made  of  smaller  materials  and  with  a  higher  percen- 
tage of  cement  than  in  preparing  the  foundation  for 
the  roadway,  and  is  given  a  surface  coating  of  cement 
mortar  which  is  commonly  formed  of  a  mixture  of  Port- 
land cement  with  finely  crushed  granite. 

Specifications  for  artificial-stone  curb  in  Washington, 
D.  C,  require  that  the  concrete  be  composed  of  I 
part  Portland  cement,  2  parts  clean  sharp  sand,  and 
3  parts  clean  broken  stone  not  more  than  I  inch  in 
their  largest  dimensions.  The  exposed  surface  of  both 
gutter  and  curb  is  to  be  coated  i£  inches  thick  with  a 
mortar  composed  of  3  parts  granulated  granite  (the 
fragments  being  of  such  size  as  to  pass  through  a 
J-inch  screen,  and  free  from  dust)  and  2  parts  cement. 

Artificial-stone  curbs  are  sometimes  made  hollow, 
and  the  interior  spaces  used  as  a  conduit  for  pipes  or 
wires.  A  variety  of  forms  are  used  for  these  cases, 
the  curb  being  usually  made  in  blocks  at  a  factory  and 
set  like  natural  stone,  the  blocks  being  commonly 
formed  in  separate  parts  which  maybe  fitted  together 
to  form  the  curb  and  removed  to  give  access  to  the 
openings.  Where  the  hollow  curbs  are  in  one  piece, 
hand-holes  are  placed  at  short  intervals  to  admit  of 
using  the  openings ;  this  may  be  done  in  case  the  con- 
duits are  to  be  used  for  wires. 

Curbs  of  burned  clay  or  brick  are  made  in  several 


200     A  TEXT-BOOK  ON   ROADS  AND   PAVEMENTS. 

forms,  both  solid  and  hollow,  and  are  frequently  used 
on  streets  paved  with  brick,  where  stone  suitable  for 
curbing  is  lacking. 

Cast-iron  curbs  are  sometimes  employed,  although 
they  have   not  come   into   use   extensively. 
They  consist  usually  of  a  casting  similar  to 
that  shown  in  section  in  Fig.  36,  which  forms 
the  face   and  top  of   the   curb,  being   open 
at  the  back  and   braced  with  ribs  at  short 
FIG     6      intervals  of  length.     They  are  held  in  place 
by  ties  attached  to  the  ribs,  and  the  backs 
are  filled  and  tamped  to  a  firm  bearing. 

Wrought-iron  plates  or  angles  are  sometimes  used  as 
a  protection  to  concrete,  or  to  resurface  a  worn  stone 
curb,  the  iron  being  fitted  to  the  face  of  the  curb  so  as 
to  form  the  exposed  surface.  Several  forms  are  used, 
and  the  process  is  patented. 

Gutters  are  commonly  formed  of  the  same  material 
as  the  roadway  pavement,  which  is  simply  extended 
to  the  curb. 

In  streets  paved  with  brick  or  granite  blocks  the 
gutter  blocks  are  sometimes  turned  lengthwise  of  the 
street,  as  shown  in  Fig.  22,  for  the  purpose  of  facilitat- 
ing the  flow  of  water  in  the  gutter.  As  already  pointed 
out,  however,  this  has  the  effect  of  making  a  continuous 
joint  between  the  pavement  and  gutter,  and  its  utility 
seems  doubtful. 

For  streets  paved  with  broken  stone  it  is  common  to 
employ  stone  gutters,  formed  of  cobblestones,  of  narrow 
flags  laid  lengthwise  of  the  gutter,  or  sometimes  of  rec- 
tangular blocks.  Such  construction  is  shown  in  Fig.  35. 
On  streets  paved  with  wood  these  gutters  may  also  be 
frequently  employed  with  advantage,  especially  where 


CITY   STREETS.  2OI 

for  any  reason  the  gutter  is  likely  to  be  kept  damp. 
In  forming  a  cobble  gutter  the  stones  are  usually  set 
upon  a  layer  of  sand  or  gravel  after  the  manner  of 
forming  a  cobble  pavement.  They  should  be  firmly 
bedded  and  form  an  even  surface. 

Cobble  gutters  are  often  used  on  village  streets 
where  no  curbs  are  set,  and  in  such  locations  where 
but  slight  expense  is  admissible  they  are  quite  satis- 
factory if  properly  constructed.  This  method  of  con- 
struction is  illustrated  in  Fig.  31. 

Sometimes  in  work  of  this  kind  a  flagstone  is  used 
for  the  bottom  of  the  gutter  and  the  sides  are  formed  of 
cobble.  This  is  preferable  as  affording  a  more  free 
channel  for  the  flow  of  the  surface  drainage. 

To  obtain  satisfactory  results  it  is  always  necessary 
that  the  foundation  be  of  sufficient  depth  and  well 
compacted,  in  order  to  prevent  the  surface  becoming 
uneven  by  the  stones  being  forced  downward  into  the 
road-bed  in  wet  weather  or  through  the  action  of  frost. 
A  layer  of  6  to  10  inches  of  gravel  or  sand  is  usually 
required. 

Where  flagstones  are  used  to  form  the  gutter,  they 
should  be  3  or  4  inches  thick,  10  to  15  inches  wide,  as 
may  be  required,  and  about  3  feet  long.  Care  is  re- 
quired in  laying  that  they  may  have  an  even  bed  and  be 
well  supported  by  the  foundation. 

Gutters  of  bricks,  or  of  stone  blocks,  are  often  used  for 
streets  upon  which  the  roadway  pavement  is  asphalt, 
on  account  of  the  liability  of  the  asphalt  being  injured 
by  dampness.  In  this  case  the  gutter  is  constructed 
by  setting  the  bricks  or  blocks  with  their  greatest 
length  along  the  street.  They  are  placed  upon  a  bed 
of  concrete,  the  same  as  is  used  for  the  foundation 


202     A   TEXT-BOOK   ON   ROADS   AND    PAVEMENTS. 

under  the  asphalt  surface,  and  the  joints  are  filled  with 
coal-tar  paving  cement,  as  in  constructing  brick  pave- 
ment. 

It  is  also  advisable  in  using  flagstone  gutters  that 
consecutive  blocks  should  have  different  widths,  differ- 
ing by  2  or  3  inches,  in  order  that  there  may  not  be  a 
continuous  joint  between  the  flagstones  and  the  pave- 
ment of  the  travelled  roadway. 

ART.  8 1.    CROSSINGS. 

On  streets  paved  with  a  smooth  hard  surface  which 
is  easily  cleaned,  such  as  brick  or  asphalt,  special  foot- 
way crossings  are  not  usually  required  or  desirable, 
unless  the  foot  travel  be  very  considerable.  On  other 
pavements,  however,  which  are  apt  to  be  rough  to 
walk  upon  or  muddy  in  bad  weather,  as  upon  stone, 
wood,  or  macadam,  footways  of  flagstones  are  com- 
monly provided,  and  form  the  most  satisfactory 
crossing. 

These  crossings  consist  of  flagstones  about  10  or  12 
inches  wide  laid  in  rows  across  the  street,  the  rows 
being  6  or  8  inches  apart  and  paved  between  with  stone 
blocks  set  in  the  ordinary  manner.  The  crossing- 
stones  are  3  or  4  feet  long,  and  at  least  6  inches 
thick  in  order  that  they  may  not  be  broken  by  the 
traffic.  They  should  be  laid  with  close  joints  and 
firmly  bedded  upon  the  foundation. 

At  street  intersections  where  the  number  of  pedes- 
trians is  large  it  is  desirable  that  the  crossing  be 
carried  across  on  the  level  of  the  top  of  the  curb 
without  leaving  a  step  at  the  gutter  crossing.  This 
may  be  accomplished  by  bridging  over  the  gutter  with 


CITY   STREETS.  203 

a  flagstone  or  iron  plate,  or  by  placing  the  outlets  for 
surface  drainage  a  few  feet  back  from  the  corner  and 
eliminating  the  gutter  at  the  corner. 

ART.  82.    STREET-RAILWAY  TRACK. 

Track  for  street  railways  upon  paved  streets  should 
be  constructed  with  a  view  to  offering  as  little  obstruc- 
tion to  ordinary  street  traffic  as  possible,  while  per- 
mitting the  ready  operation  of  the  railway.  These 
two  points  are  apt  to  conflict,  and  the  interest  of  the 
railway  company  in  the  construction  of  track  is  rarely 
identical  with  that  of  the  public  use  of  the  street. 

Track  in  streets  is  usually  constructed  of  rails  laid 
upon  cross-ties,  either  fastened  directly  to  the  ties  as 
in  the  track  of  steam  roads,  or  supported  upon  chairs 
which  serve  to  raise  the  surface  of  the  rail  to  a  greater 
height  above  the  tie,  and  in  some  cases  to  hold  the 
ends  firmly  at  the  joints.  Sometimes,  also,  the  rails  are 
laid  upon  longitudinal  wooden  stringers  placed  upon 
the  ties,  or  bolted  together  by  iron  rods  across  the 
track  without  the  use  of  ties.  Fig.  37  shows  this  sys- 


FIG.  37- 

tern   of   construction,  without  the  stringers,  the  rails 
being  set  directly  upon  the  concrete  foundation. 

Iron  ties  have  been  used  to  a  limited  extent,  and  in 
some  cases  the  rails  are  set  upon  chairs  resting  upon 
concrete. 


204     A   TEXT-BOOK   ON   ROADS  AND    PAVEMENTS. 

The  best  track  from  the  standpoint  of  the  operation 
of  the  railway  is  probably  that  formed  of  ordinary  T 
rails  laid  directly  upon  the  cross-ties  without  the  use 
of  chairs,  in  the  manner  used  for  steam  roads.  This 
form  of  construction  is,  however,  usually  unsuitable  for 
track  in  streets,  as  the  pavement  cannot  be  laid  close 
against  the  rail  at  its  upper  surface.  Where  stone  or 
wood  blocks  are  used  with  T  rails  it  is  necessary  to  cut 
away  the  corners  of  the  blocks  in  order  to  provide  a 
channel  for  the  wheel-flange.  This  has  a  tendency  to 
induce  greater  wear  under  heavy  traffic.  With  brick 
pavements  bricks  are  sometimes  moulded  of  special 
form,  with  one  corner  rounded  off,  so  that  they  may  be 
set  firmly  against  the  rail  and  still  leave  room  for  the 
wheel-flange.  This  method  has  proved  fairly  satis- 
factory in  some  places,  but  has  the  disadvantage  of 
leaving  a  corner  of  the  brick  exposed  to  wear. 

In  most  cases  where  T  rails  are  employed,  the 
rails  are  allowed  to  project  above  the  pavement  and 
form  a  serious  obstruction  to  the  ordinary  use  of  the 
street.  Even  where  the  track  is  well  constructed  and 


d 


the  pavement  originally  made  even  with  the  top  of  the 

rail,   under  any  considerable  traffic  the   wear   of   the 
pavement  near  the  rail   is  usually  rapid  and   the  rail 


CITY   STREETS. 


205 


soon  projects.     This  is  true  to  a  certain  extent  with 
any  rail,  but  more  especially  with  the  T  form. 

The  form  of  rail  now  commonly  used  in  good  con- 
struction is  that  known  as  the  girder  rail,  either  the 
ordinary  single  web-girder  rail  as  shown  in  Fig.  38,  or 
the  box-girder  rail  as  in  Fig.  39. 

The  advantage  these  rails  possess  over  the  T  rail  is 
that  the  pavement  may  be  laid  against  the  rail,  flush 
with  its  top  surface,  the  channel  for  the  wheel-flange 
being  provided  by  the  form  given 
to  the  head  of  the  rail.  The  box 
girder  is  sometimes  thought  to 
possess  an  advantage  over  the 
single  -  web  rail  from  the  fact  that 
it  affords  a  vertical  surface  against 
which  to  place  the  pavement,  and 
an  even  support  to  the  paving  blocks 
at  the  bottom  as  well  as  at  the  top,  so  that  there  is  no 
tendency  for  the  block  to  slip  under  the  flange  of  the 


FIG.  39. 


FIG.  40. 

rail.  In  the  use  of  the  single-web  rails  the  space  under 
the  flanges  may  with  advantage  be  filled  with  cement 
mortar  to  form  a  bearing  for  the  paving  block  as  shown 
in  Fig.  40. 

Where  the  paving   surface   used    is   not  too  thick, 
such  as   brick    or  asphalt,   the   track   may  usually  be 


206     A   TEXT-BOOK   ON   ROADS  AND   PAVEMENTS. 

constructed  by  spiking  the  rails  directly  to  the  ties 
as  in  Fig.  40.  If  a  thicker  surface  is  to  be  used,  as 
with  a  stone-block  pavement,  the  rails  must  be  sup- 
ported on  chairs,  unless  rails  of  extra  height  be  used 
or  longitudinal  stringers  are  placed  under  the  rails. 

Girder  rails,  as  to  the  form  of  head,  are  divided  into 
centre-bearing,  side-bearing,  and  grooved.  Of  these 
the  grooved  rail  of  form  shown  in  Fig.  38,  #,  or 
Fig.  40,  is  the  most  desirable,  considered  with  reference 
to  the  ordinary  street  traffic,  and  when  the  pavement 
is  smooth  and  kept  clean  is  satisfactory  in  use.  It  has 
been  extensively  used  in  Washington,  D.  C.  The 
objection  to  the  use  of  this  form  of  rail  is  that  the 
groove  is  likely  to  become  filled  with  dirt,  and  there- 
fore requires  constant  care  to  keep  clean,  especially 
where  the  street  is  not  maintained  always  in  good  con- 
dition. This  disadvantage  is  greater  in  cold  climates 
where  snow  and  ice  are  common  during  winter.  It 
is  also  necessary  with  this  form  of  rail  that  the  track 
be  very  accurately  gauged  in  width,  in  order  that  the 
flanges  may  properly  fit  the  grooves ;  and  it  is  desirable, 
especially  if  the  rails  be  supported  on  chairs,  that  the 
rails  be  tied  together  by  rods  as  in  Fig.  37. 

It  has  been  claimed  that  more  power  is  required  to 
move  cars  upon  rails  of  this  pattern,  even  under  favor- 
able conditions,  than  is  necessary  on  others.  The  ad- 
vantage to  the  street  traffic  of  using  these  rails  is,  how- 
ever, very  considerable.  When  placed  in  a  smooth 
pavement  which  is  made  flush  with  the  top  surface  of 
the  rail,  the  track  offers  no  obstruction  to  the  passing 
of  vehicles  over  it  in  any  direction,  and  the  inconven- 
ience and  difficulty  of  pulling  in  and  out  of  the  track 
are  avoided. 


CITY   STREETS. 


20; 


The  grooved  rail  of  form  shown  in  Fig.  38,  b,  is  some- 
times employed,  and  obviates  to  a  certain  extent  the 
difficulties  met  in  operating  track  of  the  form  just  men- 
tioned, the  groove  being  widened  at  the  top  so  that  the 
wheel-flange  may  press  the  dirt  out  at  the  sloping  side 
and  also  give  more  room  for  the  flange. 

The  side-bearing  rail  as  shown  in  Fig.  38,  c,  is  prob- 
ably more  generally  used  than  any  other.  With  this 
rail  the  flange  extends  out  on  one  side  to  form  a  channel 
for  the  wheel-flange.  It  is  more  easily  kept  clear  than 
the  grooved  form,  but  wheels  of  vehicles  readily  slip 
into  the  channel  and  leave  it  with  difficulty,  although 
when  properly  constructed  such  track  offers  no  resist- 
ances to  vehicles  crossing  it.  Fig.  41  shows  a  block 
pavement  with  track  formed  of  side-bearing  girder  rails 


FIG.  41. 

supported  by  chairs  which  are  spiked  to  the  cross-ties. 

The  centre-bearing  rail  as  shown  in  Fig.  38,  d,  forms 
the  best  track  to  operate,  because  it  keeps  clear  of  dirt 
and  offers  little  resistance  to  the  car.  It  is,  however, 
the  most  objectionable  to  the  ordinary  street  traffic,  as 
it  is  difficult  for  wheels  to  cross  it;  and  its  use  is  not 
commonly  permitted  on  streets  of  considerable  traffic. 

Many  modifications  and  combinations  of  these  forms 
are  employed  in  different  localities,  and  the  number  of 
small  variations  which  may  be  introduced  is  practi- 


208     A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

cally  endless.  In  general,  however,  nearly  all  of  the 
rails  in  common  use  belong  to  one  of  the  three  classes 
mentioned. 

In  addition  to  the  T  rails  and  girder  rails  various 
other  methods  of  construction  are  sometimes  employed. 
The  duplex  rail  is  composed  of  two  parts  rolled  sepa- 
rately and  fitting  together.  The  two  parts  break  joints, 
the  object  being  to  eliminate  the  weakness  of  the  ordi- 
nary rail-connection. 

Thin  strap-rails,  or  tram-rails  as  they  are  commonly 
called,  made  to  be  laid  upon  longitudinal  stringers  of 
wood,  are  used  to  some  extent,  but  have  in  the  main 
been  superseded  by  the  girder  forms.  They  consist 
simply  of  a  plate  of  iron  with  a  head  raised  upon  it, 
similar  in  form  to  those  already  mentioned,  the  plate 
being  laid  flat  upon  the  stringer. 

The  solid  construction  of  track  is  a  matter  of  im- 
portance upon  paved  streets,  because  of  the  difficulty 
and  expense  of  getting  at  the  track  to  make  repairs,  as 
well  as  because  of  the  disturbance  to  traffic  when  the 
pavement  must  be  removed  for  this  purpose.  The  rail- 
joints  and  tie-connections  are  therefore  matters  requir- 
ing particular  attention.  Where  no  chairs  are  used, 
the  use  of  tie-plates  to  form  a  bearing  for  the  rail  upon 
the  tie,  and  to  hold  it  securely  in  place,  is  to  be  recom- 
mended, and  will  greatly  aid  in  forming  a  rigid  track. 
There  are  a  number  of  forms  in  use  which  give  good 
results.  They  should  be  arranged  to  clamp  the  rail 
firmly  and  present  a  good  bearing  upon  the  tie.  When 
chairs  are  used,  they,  like  the  tie-plates,  should  clamp 
the  rail  firmly  and  give  good  bearing  surface.  They 
should  also  be  well  braced  for  stiffness  against  lateral 
bending. 


CITY    STREETS. 


209 


Joints,  in  the  case  of  track  formed  of  rails  laid  di- 
rectly upon  the  ties,  or  upon  wooden  stringers,  are 
usually  made  by  placing  a  plate  or  channel-bar  upon 
each  side  of  the  web  of  the  rail  ends  to  be  joined  and 
bolting  through.  The  use  of  slightly  curved  channel- 
bars  fitting  against  the  flanges  of  the  rail,  as  shown 
in  Fig.  42,  seems  to  give  good  results,  the  spring  in  the 
channels  serving  to  prevent  the 
loosening  of  the  bolts. 

Where  chairs  are  employed 
to  raise  the  rails  above  the 
ties,  joints  are  frequently  most 
satisfactorily  made  upon  long 
chairs  or  bridges  reaching 
across  the  space  between  two 
ties  and  forming  a  firm  bear- 
ing for  the  ends  of  the  rails. 

In  order  to  facilitate  keeping 
the  joints  tight  and  enable 

the  bolts  at  the  rail  ends  to  be  screwed  up  without 
taking  up  the  pavement,  joint-boxes  are  sometimes 
employed.  These  consist  of  openings  with  removable 
covers,  giving  access  to  the  bolts  at  the  ends  of  the 
rails. 

It  is  essential  to  any  good  track  construction  that 
the  track  be  well  ballasted  and  be  brought  to  an  even 
bearing  upon  the  road-bed  ;  otherwise  the  track  will 
spring  under  passing  loads  and  soon  become  uneven 
and  out  of  surface  with  the  pavement.  Gravel  or 
broken  stone  is  usually  preferred  for  ballast,  but  where 
first-class  pavements  are  employed,  founded  upon  a 
concrete  base,  the  track  should  also  be  set  in 
Crete.  This  practice  has  been  commonly 


FIG.  42. 


210     A   TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

Europe  with  good  results.  The  ballast  should  be 
firmly  tamped  about  the  ties,  which  are  preferably  of 
hewn  timber  on  account  of  the  greater  ease  of  tamping. 

The  wear  of  a  pavement  is  usually  considerably  in- 
creased by  railway  tracks  upon  the  street.  The  extent 
of  this  wear  depends  upon  the  nature  of  the  paving 
surface  as  well  as  upon  the  construction  of  the  track. 
It  is  mainly  the  difference  in  resistance  to  abrasive 
wear  between  the  rails  and  the  paving  surface  which 
causes  uneven  and  more  rapid  wear  of  the  pavement  in 
vicinity  of  the  track.  A  broken-stone  surface,  on  ac- 
count of  its  rapid  wear,  is  particularly  objectionable 
along  a  line  of  track,  and  is  very  difficult  to  keep  in 
proper  surface. 

Where  T-rail  construction  is  used  there  is  a  largely 
increased  wear  due  to  the  exposed  edges  of  the  paving 
blocks,  which  wear  rapidly  on  the  sides  and  in  the 
grooves  left  for  the  wheel-flanges.  (See  article  by  W. 
L.  Dickinson  in  Good  Roads  for  May  1894.)  With  a 
smooth  pavement  and  grooved  rails  the  wear  is  reduced 
to  a  minimum  where  the  street  is  of  sufficient  width 
to  accommodate  the  traffic  without  necessitating  the 
driving  of  loaded  vehicles  along  the  track. 

In  the  case  of  narrow  streets  or  rough  side-pave- 
ments the  use  of  the  track  for  hauling  heavy  loads 
causes  the  cutting  of  the  pavement  upon  the  outside 
of  the  track,  due  to  the  gauge  of  trucks  being  greater 
than  that  of  the  track.  This  is  especially  the  case 
where,  owing  to  the  use  of  side-bearing  or  centre-bear- 
ing rails,  the  flange  grooves  are  wide  enough  to  permit 
the  wheels  of  trucks  to  enter  them. 

Where  cable  roads  are  used  ties  are  not  employed, 
but  the  whole  structure  rests  upon  the  yokes,  which 


CITY   STREETS.  211 

pass  under  the  cable  conduit  and  sustain  the  rails  upon 
their  extremities.  The  conduits  are  usually  built  of 
concrete,  which  is  also  used  for  the  base  of  the  pave- 
ment, so  that  the  whole  structure  becomes  practically 
monolithic. 

ART.  83.    TREES  FOR  STREETS. 

It  is  always  desirable,  wherever  possible,  to  have 
streets,  at  least  those  devoted  to  residential  purposes, 
lined  with  rows  of  trees  upon  each  side,  both  for  the 
purpose  of  giving  shade  and  to  add  to  the  beauty  of 
appearance  of  the  street. 

The  most  satisfactory  way  of  arranging  trees  is  usually 
to  have  a  tree  space  between  the  sidewalk  and  the  curb 
in  which  the  trees  are  planted  in  a  straight  line  along 
the  street.  Sometimes  in  very  wide  streets  a  tree 
space  or  parking  is  arranged  in  the  middle  of  the  street, 
with  a  driveway  on  each  side.  Trees  should  be  spaced 
in  the  rows  at  such  distances  as  will  permit  each  tree 
when  fully  grown  to  spread  to  its  full  natural  dimen- 
sions,— which  usually  requires,  for  trees  ordinarily  em- 
ployed, from  25  to  40  feet. 

The  selection  of  the  variety  of  trees  to  be  used  for 
this  purpose  must  of  course  depend  upon  climatic 
and  local  conditions.  Those  which  rapidly  attain  their 
full  size  are  usually  to  be  preferred.  They  should 
have  a  graceful  form  and  make  a  good  shade,  but  the 
foliage  should  not  be  too  dense.  Evergreens  are  not 
generally  desirable  for  this  purpose.  Where  there  is 
plenty  of  room  for  their  development  the  large-grow- 
ing varieties  with  light  foliage  are  handsome  and  desir- 
able. The  size,  however,  must  be  suited  to  the  space, 


212     A  TEXT-BOOK   ON   ROADS   AND   PAVEMENTS. 

and  upon  narrow  streets,  or  where  the  trees  are  to  be 
close  to  the  buildings,  they  must  be  of  small  growth. 
The  ease  with  which  the  tree  may  be  grown  and  its 
liability  to  disease  or  to  be  affected  by  the  contamina- 
tions of  a  city  atmosphere  must  be  considered,  as  the 
conditions  under  which  street  trees  must  be  grown  are 
not  usually  favorable  to  their  best  development. 

It  is  desirable,  especially  in  cities  of  considerable 
size,  that  the  planting  and  care  of  trees  be  under  con- 
trol of  the  municipal  authorities.  Trees  may  then  be 
set  with  a  view  to  the  best  general  effect  upon  the 
street  as  a  whole,  the  selection  and  planting  of  the 
trees  may  be  properly  done,  and  the  trees  after  plant- 
ing may  be  systematically  cared  for. 

ART.  84.    ALLEYS. 

The  pavements  for  alleys  in  cities  are  constructed  in 
a  manner  similar  to  those  for  streets.  Cobblestones, 
block-stone,  brick,  and  asphalt  are  commonly  employed. 

The  maintenance  of  alleys  in  good  condition  is  a 
matter  of  no  less  importance  than  the  maintenance  of 
streets,  although  it  is  more  likely  to  be  neglected.  It 
is  of  special  importance  that  the  pavement  of  an  alley 
be  impervious,  well  drained,  and  easily  cleaned. 

The  surface  drainage  of  alleys  is  secured  either  by 
forming  the  section  as  in  a  street,  with  a  crown  at  the 
middle  and  gutters  and  curbs  at  the  sides,  or,  as  is  com- 
monly preferable  with  narrow  alleys,  by  placing  the 
gutter  at  the  middle  and  sloping  the  pavement  from 
the  sides  to  the  centre.  Where  the  gutter  is  in  the 
middle  it  is  common  to  make  the  bottom  of  the  gutter 
of  a  flagstone  15  to  18  inches  wide.  Fig.  43  shows  a 


CITY   STREETS. 


213 


centre-drained  alley  with  block-stone  pavement  upon 
sand  foundation. 

Where  the  pavement  is  cobble  or   rough  blocks  it  is 


FIG.  43. 

desirable  also  to  form  side-gutters  of  flagstones  in  order 
to  promote  ready  drainage.     Such  construction  is  rep- 


FIG.  44. 

resented  in  Fig.  44,  which  shows  a  cobble  pavement 
on  a  gravel  base,  with  curb  and  narrow  sidewalk. 


YB   1094; 


